Science of the Total Environment 429 (2012) 57–75

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Arsenic in volcanic geothermal fluids of Latin America

Dina L. López a,⁎, Jochen Bundschuh b,c,d, Peter Birkle e, Maria Aurora Armienta f, Luis Cumbal g,Ondra Sracek h,i, Lorena Cornejo j, Mauricio Ormachea k

a Department of Geological Sciences, Ohio University, 316 Clippinger Laboratories, Athens, OH, USAb Faculty of Engineering and Surveying, University of Southern Queensland, Toowoomba, Queensland 4350, Australiac KTH-International Groundwater Arsenic Research Group, Department of Land and Water Resources Engineering, Royal Institute of Technology (KTH), Teknikringen 76,SE-10044 Stockholm, Swedend Department of Earth Sciences, National Cheng Kung University, University Road, Tainan City 701, Taiwane Gerencia de Geotermia, Instituto de Investigaciones Eléctricas, Cuernavaca, Mexicof Universidad Nacional Autónoma de México, Instituto de Geofísica, Mexicog Centro de Investigaciones Científicas, Escuela Politécnica del Ejército, Sangolquí, Ecuadorh OPV (Protection of Groundwater Ltd), Bělohorská 31, 169 00 Praha 6, Czech Republici Department of Geology, Faculty of Science, Palacký University, 17. Listopadu 12, 771 46 Olomouc, Czech Republicj Departamento de Química, Facultad de Ciencias, Universidad de Tarapacá, Arica, Chile y Laboratorio de Investigaciones Medioambientales de Zonas Áridas, LIMZA, Centro de Investigacionesdel Hombre en el Desierto, CIHDE, Chilek Instituto de Investigaciones Químicas, Universidad Mayor de San Andrés, La Paz, Bolivia

⁎ Corresponding author. Tel.: +1 740 593 9435; fax:E-mail addresses: lopezd@ohio.edu (D.L. López), joc

(J. Bundschuh), birkle@iie.org.mx (P. Birkle), victoria@g(M.A. Armienta), luis.cumbal@espe.edu.ec (L. Cumbal),(O. Sracek), lorenacp@uta.cl (L. Cornejo), mrom2@kth.s

0048-9697/$ – see front matter © 2011 Published by Eldoi:10.1016/j.scitotenv.2011.08.043

a b s t r a c t

a r t i c l e i n f o

Article history:Received 18 October 2010Received in revised form 16 August 2011Accepted 16 August 2011Available online 27 January 2012

Keywords:ArsenicGeothermal systemLatin America, Volcanic fluidsGeothermal fluids

Numerous volcanoes, hot springs, fumaroles, and geothermal wells occur in the Pacific region of Latin America.These systems are characterized by high As concentrations and other typical geothermal elements such as Liand B. This paper presents a review of the available data on As concentrations in geothermal systems and theirsurficial discharges and As data on volcanic gases of Latin America. Data for geothermal systems inMexico, Gua-temala, Honduras, El Salvador, Nicaragua, Costa Rica, Ecuador, Bolivia, and Chile are presented. Two sources of Ascan be recognized in the investigated sites: Arsenic partitioned into volcanic gases and emitted in plumes and fu-maroles, and arsenic in rocks of volcanic edifices that are leached by groundwaters enriched in volcanic gases.Water containing themost elevated concentrations of As aremature Na–Cl fluids with relatively low sulfate con-tent andAs concentrations reaching up to 73.6 mg L−1 (LosHumeros geothermalfield inMexico), butmore com-monly ranging from a fewmg L−1 to tens of mg L−1. Fluids derived from Na–Cl enriched waters formed throughevaporation and condensation at shallower depths have As levels of only a few μg L−1. Mixing of Na–Cl waterswith shallower meteoric waters results in low to intermediate As concentrations (up to a few mg L−1). Afterthe waters are discharged at the ground surface, As(III) oxidizes to As(V) and attenuation of As concentrationcan occur due to sorption and co-precipitation processes with iron minerals and organic matter present in sedi-ments. Understanding the mechanisms of As enrichment in geothermal waters and their fate upon mixing withshallower groundwater and surface waters is important for the protection of water resources in Latin America.

+1 740 593 0486.henbundschuh@yahoo.comeofisica.unam.mxsrondra@yahoo.come (M. Ormachea).

sevier B.V.

© 2011 Published by Elsevier B.V.

1. Introduction

In Latin America, volcanism and geothermal systems are morecommon in the Pacific zone (Fig. 1), which is an intensively populatedregion with a high demand of potable water. The presence of As ingeothermal waters and its environmental impact has long been rec-ognized, e.g. Long Valle Caldera, USA (Wilkie and Hering, 1998); LosAzufres, Mexico (Birkle, 1998; Birkle and Merkel, 2000); LosHumeros, Mexico (González et al., 2001). The purpose of this paper

is to present a general overview of the state of As contamination aris-ing from geothermal resources in Latin America, and to identify pro-cesses that produce high As concentrations and mechanisms thatimmobilize or release As into the environment.

Geothermal activities are associated with four different settings(Chandrasekharam and Bundschuh, 2002): active volcanoes, conti-nental collision zones, continental rift systems associated with activevolcanism, and continental rifts not associated with volcanoes. In thecase of Latin America, As-rich geothermal waters are usually associat-ed with areas of active volcanism. Birkle and Bundschuh (2007b)have identified the mixing of As-rich geothermal groundwater withcold aquifers as the main environmental problem in As contamina-tion. However, in some cases, As-rich surface waters are found in riv-ers and lakes close to spring discharges (e.g. Cumbal et al., 2009), or inlakes filling volcanic calderas (e.g. López et al., 2009).

Fig. 1. Location map showing volcanoes, the ring of fire, and plate boundaries.

58 D.L. López et al. / Science of the Total Environment 429 (2012) 57–75

Evenwhen As is detected in some volcanic emissions (e.g. Signorelli,1997), it is not common to find reports of As concentrations for volcanicgases (e.g. Gemmel, 1987; Mambo and Yoshida, 1993). The way that Asis partitioned between the volcanic fluids and the magma is not wellunderstood due to lack of data in the melt and gas phase. Experimentaland theoretical work on the stoichiometry and stability of As gaseouscomplexes in the system As–H2O–NaCl–H2S at temperatures up to500 °C and pressures up to 6×107 Pa (600 bars, Pokrovski et al.,2002b) indicate that As(OH)3(gas) is the predominant As complex inboth volcanic gases and boiling hydrothermal systems. This species isproposed as responsible for the preferential partitioning of As into thevapor phase as observed in fluid inclusions from high-temperaturemagmatic-hydrothermal ore deposits (Pokrovski et al., 2002b).

With respect to As in fumarole gases, studies in Yellowstone (USA)show that toxic inorganic AsH3 is the most volatile of the inorganicspecies. Organic methylated species (CH3)2AsCl is the most common-ly found in the gas phase, followed by (CH3)3As, (CH3)2AsSCH3, andCH3AsCl2 (Planer-Friedrich et al., 2006). The degree of toxicity ofthe methylated forms is unknown.

In comparison, the behavior of sources and fate of As in geother-mal systems are better understood (e.g. Arellano et al., 2003; Birkleand Bundschuh, 2007b; Goff et al., 1986a; González et al., 2001). Indeep geothermal systems, reducing conditions prevail. Arsenic is pre-sent as As(III) and the solution is undersaturated with respect to arse-nopyrite and other As minerals (Webster and Nordstrom, 2003).These undersaturated conditions also occur for minerals containingB, F, Li, Hg, Se, and Tl. According to Webster and Nordstrom (2003),arsenopyrite is not a conspicuous mineral in geothermal systems.Birkle et al. (2010) state that the saturation state of geothermal wa-ters with respect to arsenopyrite depends on reservoir temperature.

For temperatures between 150 and 250 °C, As occurs as As-bearingpyrite rather than as arsenopyrite, or is associated with iron oxides.At higher temperatures, arsenopyrite (FeAsS) and other As-bearingminerals can be found. Equilibrium between As-bearing pyrite andfluids is responsible for the As concentrations measured in high andmoderate temperature hydrothermal systems, with local dissolutionof arsenopyrite creating more reducing conditions which are likelyto favor the precipitation of gold from hydrothermal solutions(Pokrovski et al., 2002a, 2002b).

Arsenic can be present in geothermal reservoirs as well as in springdischarge and fumarolic gases. However, the highest concentrations ofAs are found in mature NaCl waters (up to tens of thousands μg/kg)that have been in contact with the rocks for a long period of time (Birkleet al., 2010), suggesting that the increase in As concentration is due to thelonger residence time (and leaching) of the waters. Thus, As concentra-tions are considerably higher in geothermal systems occurring in volcanicrocks than in high and low enthalpy systems in sedimentary rocks. Thepath of geothermal reservoirwaters to the surface can occur in four differ-ent ways. (1) If the upflow (for example along a fault zone to the surface)is fast, with aminimal lost of conductive heat to thewallrock, the compo-sition of the dischargingwater is similar to the reservoir water producinga Na–Cl rich water, with near neutral pH, high silica content due to thelong rock–fluid interaction, sulfate concentrations lower than Cl concen-trations, and enrichment in CO2 and H2S gases. These are the matureNa–Cl waters described by Giggenbach (1988). Consequently, these wa-ters should present As concentrations close to reservoir concentrations.(2) If a vapor phase rich in H2S separates from the reservoir due to pres-sure changes, and the vapor condenses at shallower levels, acid sulfatewaters low in Cl are formed (Giggenbach, 1988). According to Birkle etal. (2010), these condensedwaters are low in As because As is partitioned

59D.L. López et al. / Science of the Total Environment 429 (2012) 57–75

preferentially into the reservoir water instead of the vapor phase, leavingwater enriched in As in the lower evaporated aquifer. (3) If the upflow ofNa–Cl waters occurs in such a way that the waters have time to oxidize,H2S is oxidized to sulfate (and different species of sulfate in solutionsuch as HSO4

−) and the pH of the water decreases. These are the acid sul-fate-Clwaters described byGiggenbach (1988) in volcanic settings. As theAs-rich waters ascend and get close to the atmosphere, oxygen-rich wa-ters can mix with the geothermal waters, or soil air in the unsaturatedzone, promoting the conversion of As(III) to As(V) and precipitation ofAs minerals if the concentrations are high enough. (4) Mixing of Na–Clwaters with shallower meteoric waters can produce the bicarbonate-rich waters described by Giggenbach (1988).

After reviewing published fluid analyses from geothermal sys-tems, Ballantyne and Moore (1988) concluded that As concentrationof reservoir fluids varies inversely with PH2S and increases with tem-perature. When vapor separation occurs in geothermal waters, Asand Cl remain in the liquid phase producing a significant positive cor-relation between these two elements (Webster and Nordstrom,2003). Arsenic concentrations in Na–Cl waters can reach high values,e.g. 50 mg L−1 in El Tatio geothermal field in Antofagasta, Chile (Ellisand Mahon, 1977; Smedley and Kinniburgh, 2002). In comparison, Asconcentrations in acid sulfate-dominated waters are usually in thelower tens of mg L−1 (Planer-Friedrich et al., 2006).

Once the waters, or gases from fumaroles, reach the atmosphere,the oxidation of As(III) occurs along the path of the fluid, convertingit to As(V). Arsenic (III) can still be present in the water under oxidiz-ing conditions in areas of the discharge zone close to hot springs if theoxidation kinetics is slow. However, the As oxidation process in thesurface waters can be accelerated by the presence of bacteria attachedto submerged macrophytes, as observed in stream waters of the east-ern Sierra Nevada (Wilkie and Hering, 1998).

In the following sections, selected geothermal sites in Latin Americawith significant As concentrations are presented. It is not possible to in-clude all sites becausemanyhavenot been investigated thoroughly and,for others, the data are not public. The chemical characteristics of thedifferent geothermal waters and their As concentrations will be com-pared to understand their evolution from the source to the dischargingpoint and in the surface environment. In Table 1, data are divided bycountry, studied site, type of discharge, and, when possible, watertype. Standard deviations are not reported because most sites onlyhave a small number of samples and the statistical distribution is un-known. A complete data set for the concentrations and field parametersused in this study, including descriptive statistics and references, is inthe Supplemental Table on the journal website.

2. México

Favorable geologic–tectonic conditions explain the widespread dis-tribution and abundance of hydrothermal systems and active volcanoesin México. Especially within the Transmexican Volcanic Belt (TMVB),dominant vertical pathways of fracture and fault systems allow the con-vective circulation of geothermal fluids. Subduction of the Rivera andCocos plates under the North American and Caribbean plates producesthousands of volcanic structures; the majority are located within twomajor arcs: the 1200-km long TMVB and the northwest-trending, SierraMadre Occidental volcanic province (SMO).More than 2300 geothermallocalities with low- tomedium-temperature conditions (28–200 °C) areidentified in 27 of 32 Mexican states (Birkle, 2007; Martínez-Estrella etal., 2005; Torres et al., 1993, 2005), most being concentrated withinthe roughly east–west trending TMVB in central Mexico (Fig. 2). Of1380 studied manifestations, there are 808 warm–hot thermal springs,526 hot wells, 25 fumaroles, 6 mud volcanoes, 11 bubbling springs,and 3 hot soils. There are 68 high enthalpy sites with temperaturesabove 150 °C (Herrera and Rocha, 1988). Unfortunately, analytical dataon the composition of thermal springs, crater lakes, and groundwaterin Mexico, including As concentrations, are limited.

In this section we present a summary of available data on As con-centrations in waters influenced by volcanic activity, thermal fluidsfrom surface manifestations, and deep geothermal reservoirs inMexico.

2.1. Mexican geothermal systems

The range of As concentrations for Mexican geothermal fluids canbe found in Table 1. A detailed listing and interpretation of As datafrom both, geothermal, and petroleum reservoirs in Mexico arereported in Birkle and Bundschuh (2009) and Birkle et al. (2010).

2.1.1. Cerro Prieto geothermal fieldThe Cerro Prieto geothermal field (Fig. 2) in northern Baja Califor-

nia is hosted in deltaic sandstone and shales of the southern SaltonSea. Geothermal fluids are sourced from depths between 800 and3000 mwith an average reservoir thickness of 1900 m. An igneous in-trusion, emplaced at a depth of 5–6 km, supplies heat to the CerroPrieto system (Elders et al., 1984), with reservoir temperaturesabove 260 °C. Arsenic data were compiled from Mercado et al.(1989) and Lippman et al. (1999) for chemical data on Cerro Prietofluids, and Portugal et al. (2000b) and Mazor and Mañon (1979) forchemical and stable isotope composition of geothermal fluids. Thedominance of sandstones in the sedimentary basin of the Cerro Prietogeothermal field explains the relatively low As concentrations of 0.25to 1.5 mg L−1 (Table 1) in reservoir fluids.

2.1.2. Las Tres Vírgenes geothermal fieldThe Las Tres Vírgenes (LTV) geothermal field is located in the middle

of the Baja California peninsula. Since 1988, nine wells have been drilledto amaximum depth of 2420 m (López, 1998). A post-Cretaceous grano-dioritic intrusion and the volcano-sedimentary Grupo Comondú form themajor hydrogeologic reservoir units of the geothermal system (Portugalet al., 2000a). Fluid compositional data and reservoir temperatures fromthree production wells at this geothermal field are reported by Portugalet al. (2000a) and Viggiano-Guerra et al. (2009). Arsenic concentrationsbetween 6.5 and 6.7 mg L−1 for LTV geothermal water (Birkle et al.,2010) are probably related to the dissolution of As from traces of primaryAs minerals or from dispersed As inclusions in the granodioritic base-ment. No primary As minerals (Quijano-León and Gutiérrez-Negrín,2003) have been documented for the LTV reservoir.

2.1.3. Los Azufres geothermal fieldLos Azufres is one of several Pleistocene silicic volcanic centers, with

active geothermal systems in the E–W trending TMVB, providing vaporand liquids froma depth between 350 and 2500 m. A 2700 m thick inter-bedding of lava flows and pyroclastic rocks of andesitic to basaltic com-position (Dobson and Mahood, 1985) provides the main aquifer withfluid flow through fractures and faults that sometimes reach the surface(Birkle et al., 2001). The NaCl-rich fluids reach maximum temperaturesof 320 °C, but temperatures of 240–280 °C are common. González et al.(2000) and Birkle (1998) present analytical results from 17 deep wellswith As concentrations between 5.1 and 28.4 mg L−1.

2.1.4. Los Humeros geothermal fieldThis geothermal field is located in the eastern part of the TMVB, with

a total of 42 wells of which 22 are currently used for electricity genera-tion. Metamorphosed carbonate forms the basement below a low-liquid-saturation reservoir, which is located at a depth between 1950and 2700m (850–100 m a.s.l.). Basalt and hornblende andesites of in-termediate permeability form the host rock of the deeper reservoir,whereas augite andesite hosts the upper geothermal reservoir (800–1700 m) (Arellano et al., 2001; Cedillo, 1999). Major andminor elemen-tal compositions of 24 fluid samples from this geothermal field arereported by González et al. (2001) and Arellano et al. (2001). The chem-ical variability in the Los Humeros reservoir (Table 1) can be explained

Table 1Arsenic concentrations, temperature, pH, and TDS for geothermal waters of Latin America. Concentrations and TDS in mg L−1. Alkalinity in mg L−1 of CaCO3. For Costa Rican waters,asterisk indicates Electrical conductivity (EC) in μS cm−1.

Site N Dischargetype

Water type MeanTemp(°C)

Max.Temp(°C)

Min.Temp(°C)

MeanpH

MaxpH

MinpH

Mean TDSor EC

Max TDSor EC

Min TDSor EC

MeanAs total

MaxAs total

MinAs total

MexicoCerro Prieto GF 3 Well Na–Cl 300 300 300 8.0 8.0 8.0 28,200 40,150 16,450 0.88 1.50 0.25Las TresVírgenes GF

2 Well Na–Cl 245 250 245 7.4 7.4 7.4 3550 3990 3110 6.60 6.70 6.50

Los Azufres GF 14 Well Na–Cl 275 329 202 7.3 7.9 6.5 7551 11,040 5410 24.00 29.50 5.10Los Humeros GF 17 Well Na–Cl to Na–HCO3–

SO4–Cl304 330 280 7.7 8.7 6.6 2419 2740 1070 21.00 73.60 1.90

El Chichón volc. 1 Spring Na–Cl 58 2.2 0.15El Chichónvolc.

1 Craterlake

Na–Ca–Cl–SO4 36 2.3 0.17

Colima volc. 1 VGC 828 0.53Popocatépetlvolc.

1 Craterlake

Mg–SO4–Cl 21 1.5 1.20

Popocatépetlvolc.

2 Spring Ca–Mg–HCO3–SO4, Mg–Ca–Na–HCO3–SO4

21 26 17 6.7 6.7 6.7 533 964 101 0.04 0.05 0.03

GuatemalaTecuamburro GF 1 Hot

springCa–Mg–SO4 77 2.5 0.10

Tecuamburro GF 3 Hotspring

Na–Cl to Na–HCO3–Cl 59 96 39 6.5 7.0 6.0 0.80 2.00 0.10

Zunil GF 2 Hotspring

Na–HCO3–Cl Mg–Ca–Cl–SO4 68 74 61 7.5 7.6 7.4 1177 1683 670 0.31 0.34 0.27

Zunil GF 8 Well Na–Cl 239 278 95 7.4 8.4 5.7 2826 4222 212 4.83 12.34 0.14Moyuta GF 2 Hot

springNa–HCO3–Cl–SO4 71 75 67 7.2 7.2 7.2 5.75 6.60 4.90

HondurasAzacualpa GF 1 Hot

spring115 0.07

Pavana GF 1 Hotspring

102 0.11

Platanares GF 15 Hotspring

Na–HCO3–SO4 98 100 95 8.7 9.1 8.0 0.94 1.26 0.68

El SalvadorAhuachapan GF 7 Cold

springCa–Mg–HCO3–SO4 to Mg–Na–Ca–HCO3

29 32 19 6.6 7.0 6.1 0.04 0.09 0.01

Ahuachapan GF 1 Hotspring

Ca–Na–SO4 76 7.3 0.01

Ahuachapan GF 1 CraterLake

Mg–Ca–HCO3 19 6.4 0.21

Ahuachapan GF 3 Domesticwell

Na–Mg–Ca–HCO3, Na–HCO3–Cl

32 33 30 6.9 7.5 6.6 0.08 0.09 0.08

Coatepeque C. 2 Hotspring

Na–Cl 66 6.7 2.3 3.1 1.5

Coatepeque C. 7 Lakewater

Na–Cl–SO4 24 8.5 0.80 0.50 1.19

Ilopango C. 12 Lakewater

Na–KHCO3–Cl 0.53 0.78 0.15

Berlin GF 6 Hotspring

HCO3, HCO3–SO4 58 96 38 6.7 7.6 5.3 0.04 0.16 0.01

Berlin GF 1 Hotspring

Cl 101 8.1 0.33

Berlin GF 5 Well Na–Cl 6.4 7.0 6.1 11.7 16.7 7.8

NicaraguaMonte Galan 3 Hot

springNa–HCO3–SO4 to Na–Mg–Ca–HCO3–SO4–Cl

44 47 41 6.6 6.8 6.4 1126 1220 970 0.11 0.12 0.11

Momotombo GF 3 Well Na–Cl 80 100 39 8.3 8.4 8.1 4932 7026 3853 2.09 2.65 1.74Cerro Negro volc. 7 VGC Mg–Cl to Ca–Mg–Cl–SO4 263 315 170 1.5 2.0 1.2 0.07 0.10 0.03Masaya volc. 3 VGC Na–Cl to KCa–SO4–Cl 112 150 85 3.7 4.4 2.7 0.16 0.40 0.04Telica volc. 1 VGC K–Cl 150 2.6 0.08Momotombovolc.

5 VGC Na–Mg–Cl to Na–Mg–Cl–SO4 532 666 471 0.8 0.9 0.7 0.30 0.49 0.23

Costa RicaMiravalles GF 13 Well Na–Cl 234 242 227 7.0 7.8 4.8 12,728* 15,150* 8940* 25.42 29.13 11.86Miravalles GF 11 Hot

springNa–HCO3 to Mg–Ca–HCO3–SO4

47 89 34 4.9 7.3 2.0 1536* 4840* 260* 0.57 4.56 0.01

Miravalles GF 4 Coldspring

Ca–HCO3–SO4 toCa–Mg–HCO3

21 24 14 6.8 7.2 6.6 228* 300* 140* 0.01 0.01 0.01

60 D.L. López et al. / Science of the Total Environment 429 (2012) 57–75

Table 1 (continued)

Site N Dischargetype

Water type MeanTemp(°C)

Max.Temp(°C)

Min.Temp(°C)

MeanpH

MaxpH

MinpH

Mean TDSor EC

Max TDSor EC

Min TDSor EC

MeanAs total

MaxAs total

MinAs total

Costa RicaRincón de laVieja GF

4 Well Na–Cl 247 276 229 6.5 7.8 5.7 10,240* 12,850* 8000* 9.90 13.00 5.99

Rincón de laVieja GF

8 Hotspring

Mg–Cl–SO4 to Ca–Mg–SO4–HCO3

47 91 26 5.6 7.6 2.2 2122* 5850* 190* 0.02 0.05 0.01

Rincón de laVieja GF

2 Hotspring

Na–Cl 71 71 71 6.1 6.1 6.1 9520* 9580* 9460* 10.74 10.85 10.63

Rincón de laVieja GF

2 Coldspring

NaCl to Ca–Mg–SO4–HCO3 27 27 27 6.9 7.8 6.0 295* 380* 210* 0.07 0.13 0.01

Poas volcano 2 VGC 8.6 14.7 2.6Irazú volc. 1 VGC 0.013Rincón de laVieja volc.

1 VGC 0.05

HornillasMiravallesvolc.

1 VGC 0.05

EcuadorEl Carchi 4 Hot

springCl to Cl–HCO3 39 54 31 6.2 7.3 4.8 581 0.277 0.684 0.002

Imbabura 4 Hotspring

Cl–HCO3 41 44 39 7.3 8.0 6.5 3187 3700 2710 0.683 0.974 0.004

Pichincha 7 Hotspring

HCO3–Cl 35 42 29 6.8 7.4 6.3 1229 2310 412 0.184 0.405 0.048

Cotopaxi 3 Hotspring

HCO3–Cl 24 34 19 6.5 6.7 6.4 0.032 0.045 0.012

Tungurahua 5 Hotspring

HCO3–Cl, Cl 41 54 28 7.3 8.3 6.4 0.049 0.114 0.004

Papallacta LakeBasin

7 Hotspring

41 63 14 7.0 8.2 6.2 1759 3500 162 4.19 7.85 1.09

BoliviaPoopo's LakeBasin

16 Hotspring

Na–Cl to Na–HCO3–Cl 56 75 40 7.0 8.3 6.3 0.023 0.065 0.008

Coipasa aUyuni

1 Coldspring

Na–Ca–SO4–Cl–HCO3 17 8.3 622 0.03

ChileCaritaya river 3 River Cl 8.1 8.1 8.1 2287 3127 1620 2.66 3.20 2.26Amuyo's lakes 3 Lake Cl 6.9 6.9 6.8 11476 12481 10780 10.96 12.69 9.58El Tatio 1 Geyser Na–Cl 87 7.2 2630 7.60El Tatio 2 Hot

springNa–Cl 87 6.3 6.4 6.2 9590 28.5 30.1 27.0

El Tatio 1 River Na–Cl 22 7.7 6620 21.00

61D.L. López et al. / Science of the Total Environment 429 (2012) 57–75

by vertical groundwater zoning, with lower concentrations of As fluids(3.9–7.9 mg L−1) in the liquid-dominated shallow part of the reservoir(1330–1755 m b.s.l.), but the highest and most heterogeneous As con-centrations (1.9–73.6 mg L−1) in the deeper (1985–3060 m b.s.l.),two-phase reservoir zone.

2.2. Méxican active volcanoes

Although eruptions of 15 volcanoes (11 polygenetic and 3 mono-genetic) have been reported in historical times in México (De laCruz-Reyna, 2001), As concentrations have mainly been determinedin gases and waters influenced by the activities of only three volca-noes: Popocatépetl, El Chichón, and Colima.

2.2.1. Popocatépetl volcanoPopocatépetl is an andesitic–dacitic stratovolcano 5454-m-high lo-

cated in the TMVB about 70 km southeast of Mexico City (Fig. 2). Ithas an elliptical summit crater, which contained a roughly circularlake (SEAN, 1986) that disappeared in 1994 before the onset of currentactivity (Armienta, 2008). Hydrogeochemical studies on the lake andsprings discharging around or at the volcanic edifice (Armienta et al.,2000, 2008a, 2008b; Inguaggiato et al., 2005; Segovia et al., 2003)have not been focused on As, resulting in little concentration data.

In 1863, Lefort reported the presence of As traces in the crater lakewaters of Popocatépetl (Clarke, 1911). Samples collected in February1994 before the disappearance of the most recent lake contained1.2 mg L−1 of As (Werner et al., 1997). Arsenic ranged from b0.001to 0.053 mg L−1 in spring samples collected in 1994 and 1995 at var-ious distances from the Popocatépetl summit (mean=0.021 mg L−1,n=91) with the highest As concentrations furthest from the volcano(about 46 km south); two samples had also the highest temperatureand Cl concentrations (Werner et al., 1997). Segovia et al. (2002,2003) reported low As concentrations (from 0.001 to 0.003 mg L−1,with mean equal to 0.002 mg L−1, n=6) in springs dischargingfrom the volcano flanks. Low As concentrations were also measuredin aqueous leachates of tephras erupted in 1996, 1997 and 1998. Upto 0.08 mg kg−1 were obtained, with a mean equal to 0.019 mg/kg,n=17 samples (Armienta et al., 2002). Alkaline traps installed to col-lect gases in 1994 showed a pulse of As in the first months of that year(with a concentration rate up to 0.02 mg kg−1 day−1) (Werner et al.,1997). The SO2 emissions measured by COSPEC (Werner et al., 1997)was estimated as an average As flux of 0.10 t day−1.

2.2.2. El Chichón volcanoThis is an isolated volcano located 1100 m a.s.l. at the NWend of the

Chiapas Volcanic Arc (CVA) (Fig. 2). The basement rocks are Cretaceous

Los Humeros

Las Tres Vírgenes

Los Azufres

Primavera

Cerro Prieto

Colimavolcano

Popocatépetlvolcano

El Chichónvolcano

Fig. 2. Location of potential hydrothermal sites (black dots), geothermal fields, and described volcanic sites on Mexican territory (modified after Birkle, 2007).

62 D.L. López et al. / Science of the Total Environment 429 (2012) 57–75

evaporites and limestones, with interbeds of epiclastic early Cenozoicsandstones and limestones (Canul andRocha, 1981). The volcanic struc-tures and deposits are calcalkaline in composition with a medium tohigh content of potassium. El Chichón produced elevenmajor eruptionsin the last 8000 years (Espindola et al., 2000;Mora et al., 2007). Compo-sition of the crater lakewater has been studied since 1983 by various re-searchers (Armienta and De la Cruz-Reyna, 1995; Armienta et al., 2000,2008a; Casadevall et al., 1984; Rouwet et al., 2008; Taran et al., 1998,2008; Tassi et al., 2003). However, only some of the publications reportAs concentrations. Varekamp et al. (1984)measured up to 120 mg kg−1

(mean=49mg kg−1, n=11) in aqueous ash-leachates from the 1982eruption. Variable As concentrations (from 0.032 to 0.17 mg kg−1)were determined inwater samples from the crater lake collected at var-ious dates. Springs flowing from its flanks contained a wide range in Asconcentrations (from 0.01 to 0.146 mg L−1), with a mean of0.044 mg L−1 (n=7). The highest concentration corresponded to anacidic (pH=2.18) highly-saline (TDS N15,000 mg kg−1), Na–Ca–Cl-type spring apparently influenced by amagmatic source with contribu-tion from a deep brine (Armienta, 2008; Taran et al., 2008).

2.2.3. Colima volcanoColima volcano, located in thewestern portion of the TMVB (Fig. 1), is

an andesitic stratovolcano rising nearly 4000 m a.s.l.. With a historical re-cord of 25 eruptions since 1560, it is the most active volcano in Mexico(De la Cruz-Reyna, 1993). Chemical studies of fluids from fumaroles,lakes, rivers, and springs related with the activity of the volcano havebeen performed for about fifteen years (Armienta and De la Cruz-Reyna,1995; Connor et al., 1993; Taran et al., 2000; 2001). Nevertheless, onlydata on As concentrations from gas emissions are available. Condensates

collected at Colima volcano in 1997 contained up to 0.525 mg kg−1 As(mean=0.439 mg kg−1, n=3).

3. Central America

Volcanic- and tectonic-related hydrothermal systems are commonin Central America because of the effects of two plate boundaries: theMotagua–Polochic transform faults that define the boundary betweenthe North American and the Caribbean Plates and the subduction zonedefining the boundary between the Cocos and Caribbean Plates(Fig. 3). Acharya (1983) suggested, as seen in Central America, that geo-thermal systems in the circum-Pacific rim are situated near the ends ofplate boundary segments or in transverse zones that divide plates inblocks that have a length between 100 and 1000 km, corresponding tolateral breaks of the underthrusting plates. Fig. 3 shows the main geo-thermal systems in Central America. Location, lithology, local productionhistory, and conceptual hydrogeologic model of the main geothermalsites in Central America are given in Birkle and Bundschuh (2007a), andthe corresponding chemical and isotopic fluid composition in Birkle andBundschuh (2007b). Arsenic concentrations in geothermal systems havebeen determined only in a few geothermal systems of the region (e.g.Platanares, Goff et al., 1986a; Zunil, Adams et al., 1991; Berlin, Jasmin etal., 2005). Selected geothermal systems with As data are discussed next.

3.1. Guatemala

Thirteen geothermal areas of interest have been investigated inGuatemala (Lima Lobato et al., 2003) (Fig. 3). Two geothermal sys-tems are in exploitation, Zunil and Amatitlan with 28 MWe and 30

45 46

Caribbean Sea

Honduras

Guatemala

Nicaragua

Mexico Belize

El Salvador

Costa Rica

Pacific Ocean

N

250 km

1

2 3

4 5 6

7

8 9 10

11 12

13 14

15

16

17

18

19

20 21 22 23 24 25 26

27

28 29

30 31

32

33

34 35

36 37 38

39

40

41

42 43

44

= Faults

= Behind arc volcano

= Arc volcano

1-San Marcos 2-Zunil 3-Atitlan 4-Palencia 5-Amatitlan 6-Tecuamburro 7-Motagua 8-Ayarza 9-Retana 10-Ixtepeque Ipala 11-Los Achiotes 12-Moyuta 13-Totonicapan 14-Azacualpa 15-Pavana

16-Platanares 17-El Olivar 18-Sambo Creek 19-San Ignacio or La Tembladera 20-Ahuachapan 21-Coatepeque Caldera 22-San Salvador volcano 23-Ilopango Caldera 24-San Vicente 25-Berlin 26-Chinameca 27-Olomega Lake hotsprings 28-Casita-San Cristobal 29-Cosiguina 30-El Hoyo-Monte Galan

31-El Najo-Santa Isabel 32-Isla de Ometepe 33-Managua-Chiltepe 34-Masaya-Granada-Nandaime 35-Nagarote-La Paz-Centro 36-San Jacinto-El Tizate 37-Tipitapa 38-Momotombo 39-Fortuna-Poco Sol Caldera 40-Irazu-Turrialba 41-Miralvalles 42-Orosí-Cacao 43-Poás 44-Porvenir-Platanar 45-Rincón de la Vieja 46-Tenorio

Fig. 3. Geothermal fields and volcanoes of Central America.

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MWe proven capacity, respectively. The geothermal systems ofTecuamburro (Goff et al., 1989), Zunil (Adams et al., 1991), andMoyuta (Goff et al., 1991) contain As concentrations in the dischargewaters.

3.1.1. Tecuamburro geothermal fieldTecuamburro geothermal field located in western Guatemala is re-

lated to the volcanowith the samename (Fig. 3). Tecuamburro is a com-posite andesitic volcano of Pleistocene age (Duffield et al., 1992) thathas an abandoned sulfur mine in the summit area and an acidic lake

0.5 km in diameter (Laguna Ixpaco) in its crater. Acid sulfate watersoccur at Laguna Ixpaco, steam-condensate waters occur north of thevolcano, and neutral Cl waters discharge at springs close to Rio LosEsclavos at the east side of the volcano (Goff et al., 1989). Some coldsprings reflecting shallow circulation are also identified in the area. Ar-senic concentrations are low (b0.05 mg L−1) in the cold springs, steam-condensate springs, and in the acid sulfate waters. Only Laguna Ixpacohas As concentrations slightly higher (0.10 mg L−1). In comparison,the neutral Cl springs exhibit As concentrations ranging from 0.1 to2.0 mg L−1, with a mean of 0.8 mg L−1 (n=3; Goff et al., 1989).

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3.1.2. Zunil geothermal fieldZunil geothermal field is located within the volcanic complex

formed by Santa Maria, Cerro Quemado, and Zunil volcanoes, about120 km to the west of Guatemala City. The area falls within a pro-posed caldera ranging in age from Tertiary to Pleistocene (Foley etal., 1990) with the caldera wall and related faults transferring thefluids and heat (Bennati et al., 2011). Only two of sixteen fumarolesand warm and hot springs sampled show detectable As concentra-tions (0.27 and 0.34 mg L−1) and they correspond to two hot springswith higher concentrations of bicarbonate (665 and 210 mg/L, re-spectively, Adams et al., 1991). In comparison, the majority of thegeothermal wells (8 out of 12 sampled wells) have high concentra-tions of As, ranging from 0.14 to 12.34 mg L−1 (mean=4.83 mg L−1),while pH ranges from 5.7 to 8.4. Cold springs display no detectable Aslevels.

3.1.3. Moyuta geothermal fieldThis volcano lies in a west-trending belt of domes and lava flows of

andesitic–dacitic composition overlapping the southern boundary ofthe Jalpatagua Graben (Williams et al., 1964) in eastern Guatemala,close to the El Salvador–Guatemala border. Chemistry of the thermaland non-thermal waters allows the identification of four different typesof waters (Goff et al., 1991): a) dilute Ca-bicarbonate non-thermal coldsprings, b) fumaroles and steam-heated bicarbonate-rich springs at thenorth and south flanks, and c) acidic sulfate-rich springs in the fumaroleareas, where the As concentration is below the detection limit; and d) Cl-rich springs that occur at the lower elevations along rivers and have pHranging from 6.5 to 7.2. As concentrations, as well as F, B. and Li, aredetected only in two of four springs sampled (4.9 and 6.6 mg L−1).

3.2. Honduras

Honduras has only a relatively short boundary with the PacificOcean (and the Cocos–Pacific plate boundary) at the FonsecaGolf. How-ever, due to the presence of several grabens and extensional faults, Hon-duras has a large number of thermal sites. Finch (1987) mentioned theexistence of 125 thermal springs (temperature N30 °C) and 56 addi-tional sites that have not been verified. Goff et al. (1986b) studied thegeochemistry of springs in six geothermal sites in Honduras anddetected As in three of them – Azacualpa, Pavana, and Platanares(Fig. 3) –with As concentrations of 0.07, 0.11, and 1.26 mg L−1, respec-tively, based on one sample from each site. Arsenic was not detected inthe other three sites (El Olivar, Sambo Creek, and San Ignacio (Fig. 3)).More extensive geochemical work has been done at three sites: Plata-nares, La Tembladera, and Azacualpa geothermal systems (Aldrich etal., 1987; Eppler et al., 1987; Laughlin, 1988), but As concentrationswere reported only from the Platanares geothermal field.

3.2.1. Platanares geothermal fieldThis geothermalfield is located about 16 kmwest of the city of Santa

Rosa de Copán in west-central Honduras. Silicic tuffs and andesite lavaflows are underlain by red beds of Cretaceous age (Laughlin, 1988).Wa-ters discharge in many hot springs of alkaline-Cl composition, whichhave equilibrated at greater depths with sedimentary rocks at tempera-tures of 200–245 °C as indicated by chemical and isotopic geotherm-ometers (Birkle and Bundschuh, 2007b; Janik et al., 1991). Aconceptual model of a high temperature fluid ascending through faultsand cooling by conduction to form a 160–165 °C shallow reservoir hasbeen proposed (Janik et al., 1991). Hot springsmay be sourced fromboil-ing and steam loss of the parental fluid. Goff et al. (1986a) and Janik et al.(1991) studied the hydrogeochemistry and divided thewaters into threetypes: end-member geothermalwaters, coldwaters, andmixed geother-mal waters. Arsenic was not detected in the cold waters, in the geother-mal waters, As concentrations ranged from 0.68 to 1.26 mg L−1

(mean=0.94 mg L−1, n=15), with pH values of 8.0 to 9.0.

3.3. El Salvador

El Salvador is characterized by numerous volcanoes (Williams andMeyer-Abich, 1955), fumaroles, and hot springs (Fig. 3). Many faultsand contacts between different rock types channel the circulation of hy-drothermal fluids surrounding themagmatic chambers of active aswellas dormant volcanoes (e.g. López et al., 2004; Pérez et al., 2004). Themost significant geothermal systems in El Salvador are aligned fromwest to east (Fig. 3): Ahuachapán and Chipilapa hydrothermal fields,Cerro Pacho and thermal springs on the shore of Coatepeque calderalake, several bicarbonate springs at San Salvador volcano, possible sub-aqueous seeps in Ilopango caldera lake, the hydrothermal fields ElObraje and SanVicente associatedwith SanVicente volcano; Berlín geo-thermal field near Tecapa Volcano; the Chinameca hydrothermal field;and thermal springs at the shores of Olomega lake.

Geothermal energy has been exploited in El Salvador since 1975 attwo sites: Ahuachapán and Berlín. There are some data on As in geo-thermal fields (e.g. Jasmin et al., 2005; Sullivan, 2008). Some of thedata collected by industry have been made available to the publicthrough ISOHIS (Isotope Hydrology Information System) as part ofthe Global Network of Isotopes in Precipitation (GNIP) formed bythe International Atomic Energy Agency (IAEA).

3.3.1. Ahuachapán geothermal fieldThe Ahuachapán and Chipilapa geothermal fields are associated with

the Concepción de Ataco caldera and the volcanoes Laguna Verde andLaguna de las Ninfas. The Ahuachapán andesites are recognized as thepermeable unit comprising most of the reservoir (Electroconsult,1984). The NNW-SSE striking faults present in the area are producedby the movement of the Caribbean Plate parallel to the Motagua–Polochic–Jocotán transverse fault that separates the Caribbean andNorth American plates (González Partida et al., 1997). This fault systemis recognized as the main conduit for fluid transfer at the Ahuachapángeothermal field. Data for seven cold springs, one hot spring, one craterlake, and three domesticwells exist in the ISOHIS database (InternationalAgency for Atomic Energy, IAAE). The waters discharged at Ahuachapánfrom cold springs have As concentrations from 0.01 to 0.09 mg L−1

(mean=0.04 mg L−1), with pH from 6.1 to 7.3 (mean=6.7). Domesticwell waters at Ahuachapán vary in As concentrations from 0.08 to0.090 mg L−1 (mean=0.08 mg L−1), with pH from 6.6 to 7.5(mean=6.9). The waters of the crater lake have the highest concentra-tion of As (0.21 mg L−1)with a pHof 6.4. Arsenic data for the geothermalwells under exploitation have not been published.

3.3.2. Coatepeque geothermal fieldCoatepeque caldera forms part of the Santa Ana–Izalco–Coatepeque

volcanic complex located inwestern El Salvador (Fig. 3). The caldera liesat the border of a hypothesized pull-apart structure that links twomajorsegments of the El Salvador Fault Zone (ESFZ) (Agostini et al., 2006).This caldera formed as a result of volcano collapse 50–70 thousandyears ago (Pullinger, 1998). Intracaldera activity has continued at Coat-epeque, and the Cerro Pacho dome located to the southwest of the cal-dera, probably the most recently emplaced structure. Hydrothermalactivity is visible at this dome as well as at hot springs discharging atthe lake's shore in the same area. Arsenic data for hot springs and forlake waters are reported in McCutcheon (1998). For the hot springs,two values for As concentrations are reported: 3.1 and 1.5 mg L−1. Asranges from 0.09 to 1.19 mg L−1 (mean 0.47 mg L−1, n=7) in the lake.

3.3.3. Ilopango LakeWilliams and Meyer-Abich (1955) stated that Ilopango caldera

formed via three distinct collapse episodes that produced violent volca-nic eruptions. The last caldera collapse generated the Tierra BlancaJoven deposit and occurred approximately 1600 years ago (A.D. 429)forming the basin that now contains the lake (Hart, 1981). Warmerwater has been reported by fisherman in the southern area of the lake.

65D.L. López et al. / Science of the Total Environment 429 (2012) 57–75

In Ilopango Lake, As concentration range from 0.15 to 0.78 mg L−1

(mean=0.53, n=12) in the waters to 5.6 to 103.4 mg kg−1 in the sedi-ments (Ransom, 2002; López et al., 2009). For the sediments of Ilopango,statistically significant correlations were found for Li vs. As and for B vs.As (López et al., 2009), consistent with their common volcanic origin.However, the points of maximum concentrations in the sediments forAs and B differ from those in the water (López et al., 2009). The areaswith the highest concentrations of B and As in the water are located inthe southern lake, corresponding to sediment samples with the lowestAs and B concentrations. Previous investigations in soil gases in this cal-dera included samples along the perimeter of the lake (López et al.,2004). Comparison between these two studies showed that the southernpart of the lake presents the highest emissions of carbondioxide and con-centrations of radon, following the general trend of the concentrations ofAs and B in the water (López et al., 2009), suggesting leaching from thesediments as one possible mechanism of As enrichment in the waters.

Ilopango Lake discharges into the Desague River, which is a tribu-tary of Jiboa River. Arsenic data from these two rivers (Esquivel,2007) showed a clear attenuation of As concentration with distancefrom the lake, suggesting sorption or co-precipitation processes thattransfer the As from the water to the sediment or organic matter.

3.3.4. Berlín geothermal fieldThis The Berlín hydrothermal field is approximately 100 km east-

southeast of San Salvador (Fig. 3) on in the northern flank of the basal-tic–andesitice Tecapa volcano. This geothermal field is located withinNW–SE trending faults that form a 5 km-wide graben system thatformed at the same time than the Berlín caldera collapse (100 Ka). Pre-vious studies performed in the north-central area of the Berlín field sug-gest the presence of a groundwater system consisting of three principalaquifers (Shallow, Intermediate, and Deep) (Montalvo and Axelsson,2000), with the deep aquifer as the geothermal reservoir. Data forsprings and domestic wells in the Berlín area are listed and reported inthe ISOHIS data base aswell as other sources (e.g., Sullivan, 2008). How-ever, only the springs with temperatures higher than around 33 °C con-tain As concentrations higher than 0.025 mg L−1. The cold sources(n=44, Sullivan, 2008) have concentrations of As ranging from 0.001to 0.025 mg L−1 (mean=0.010 mg L−1). For the hot springs, As con-centrations range from 0.01 to 0.33 mg L−1 (mean=0.08 mg L−1).Similarly, for the domestic wells, As concentrations are low, rangingfrom 0.004 to 0.042 mg L−1 (mean=0.020 mg L−1, n=16). Arsenic inthe Na–Cl waters of the geothermal wells range from 7.8 to16.7 mg L−1 (mean=11.7 mg L−1) for five sampled wells (Jasmin etal., 2005). For these wells, pH ranged from 6.1 to 7.0 (mean=6.4).

3.4. Nicaragua

Nicaragua has many geothermal fields and volcanoes, with theMomotombo geothermal field in exploitation since 1983. The geo-thermal resources identified in Nicaragua are (Fig. 3): Casita-San Cris-tobal, Cosigüina, El Hoyo-Monte Galán, El Ñajo-Santa Isabel, Isla deOmetepe, Managua-Chiltepe, Masaya-Granada-Nandaime, Momo-tombo, Nagrote-La Paz Centro, San Jacinto-El Tizate, and Tipitapa(Battocletti, 1999; Birkle and Bundschuh, 2007a). However, As datafrom these sites are limited.

3.4.1. Momotombo and Monte Galán geothermal fieldsTheMomotombo volcanic complex consists of several small volcanic

cones and a caldera.Momotombo volcano is located on the eastern edgeof the 4.5 km diameter Monte Galán caldera (Goldsmith, 1975). Twomajor fault systems are present in this area, the NW–SE fault systemthat forms the PuertoMomotomboGraben and the NE–SW fault system(e.g. the Puerto Sandino fault). These two fault systems are the prefer-ential flow paths for hydrothermal waters. The westernmost, NE–SWtrending fault seems to be the main conduit for the upwelling fluids ofthe caldera (Porras et al., 2007).

In a comprehensive study of surface and groundwaters of theMana-gua area, Parello et al. (2008) report compositions of three spring sam-ples collected between the Monte Galan Caldera and the Asososca deLeon volcano, close to the Momotombo volcano, as well as three sam-ples from wells in the Momotombo geothermal field. The first threesamples correspond to bicarbonate-rich waters with As ranging from0.11 to 0.12 mg L−1 (mean=0.11 mg L−1), and pH from 6.4 to 6.8. Incomparison, the geothermal wells from Momotombo discharge Na–Clwaters that have As concentrations ranging from 1.7 to 2.7 mg L−1

(mean=2.1 mg L−1), with pH from 8.1 to 8.4 (mean=8.3).

3.4.2. Nicaraguan volcanoesArsenic has beendetermined in condensates fromgases of several vol-

canoes in Nicaragua. For Cerro Negro, Momotombo, and Masaya volca-noes, Gemmel (1987) reports As concentration ranges of 0.03 to0.10 mg L−1 (mean=0.07 for 7 samples), 0.23 to 0.49 mg L−1

(mean=0.30 for 5 samples with detectable As out of 8 samples), and0.04 to 0.40 mg L−1 (mean=0.16 mg L−1, for 3 samples), respectively,and 0.08 mg L−1 at Telica volcano (one sample).

3.5. Costa Rica

Several geothermal resources have been identified in Costa Rica:Fortuna-Poco Sol Caldera, Irazú-Turrialba, Miravalles, Orosí-Cacao,Poás, Porvenir-Platanar, Rincón de la Vieja, and Tenorio (Battocletti,1999). One of these areas, Miravalles, has152 MW installed capacityin three different sectors of the volcano.

3.5.1. Rincón de la Vieja geothermal fieldRincón de la Vieja geothermal system is hosted in predominantly

andesitic rocks. Springs in this system are characterized by low Asconcentrations with the exception of one area that discharges Na–Clwaters similar to the geothermal reservoir (springs Salitral Norte 1and 2) (Birkle and Bundschuh, 2007b), with 10.6 and 10.9 mg L−1

As concentrations, respectively (Hammarlund and Piñones, 2009).Arsenic concentrations in the geothermal reservoir range from 6.0to 13.0 mg L−1; with a mean equal to 9.9 mg L−1 (n=4). The othergeothermal springs are conductively-heated meteoric waters (Birkleand Bundschuh, 2007b) with As concentrations ranging from 0.005 to0.13 mg L−1 (Hammarlund andPiñones, 2009;Hammarlund et al., 2009).

3.5.2. Miravalles geothermal fieldThis field in western Costa Rica is an andesitic reservoir with As

concentrations ranging from 11.9 to 29.1 mg L−1 (n=13) with an av-erage concentration of 24.4 mg L−1 (Hammarlund and Piñones,2009). Only one spring with acid pH (2.3) of Ca–SO4–type showshigh As concentrations of 4.56 mg L−1, suggesting this water containsa magmatic component. Two Na–Cl–bicarbonate-type springs showhigh As concentrations (Na–Cl Salitral Bagaces 1 and 2) with6 mg L−1 of arsenic (Antonio Yock, pers. Commun.), and high Naand Cl concentrations of 2100 and 2600 mg L−1, respectively (Birkleand Bundschuh, 2007b). These two springs should not be confusedwith the Na–HCO3 spring Salitral Bagaces reported by Hammarlundand Piñones (2009) from the same site, which has low electrical con-ductivity and an As concentration of only 0.188 mg L−1. This thermalspring (60 °C) has changed its characteristics from a permanent to anintermittent spring and sampling has become only occasionally possi-ble. The other geothermal springs are likely generated by condensedsteam or conductively-heated meteoric water which is reflected in theirlower As concentration, ranging from 0.001 to 0.281 mg L−1 (n=10),with a mean of 0.112 mg L−1 (Hammarlund and Piñones, 2009).

3.5.3. Costa Rican volcanoesThe concentration of As in gases emitted by several volcanoes in

Costa Rica, as has been determined by several authors. The concentra-tion of As in the gases of Poás volcano is been reported as 2.55 mg L−1

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(Signorelli, 1997), and 14.71 mg L−1 (Naranjo fumarole, Bundschuhet al. unpublished data). For Irazú, Rincón de la Vieja, and HornillasMiravalles, Signorelli (1997) reports As concentrations for gas con-densates as of 0.013, 0.050, and 0.050 mg L−1, respectively.

4. North-Central and Northeastern Andean Regions of Ecuador

Ecuador has many volcanic centers and geothermal systems. Geo-thermal waters with high As concentrations are located in the north-central Andean region of Ecuador (Fig. 4) and the basin of PapallactaLake in Quijos County (Fig. 5).

4.1. North-Central Region

This area is located between 1°11 N and 1°30S, and includes fiveprovinces: El Carchi, Imbabura, Pichincha, Cotopaxi, and Tungurahua(Fig. 4). This region covers 30,134 km2 and has a population ofmore than 3,600,000 inhabitants (Instituto Ecuatoriano de Estadísticay Censo, 2001). The geology of this region is formed predominantlyby andesitic to rhyolitic volcanic material (Baldock, 1983). For a com-plete chemistry of the different springs, see Cumbal et al. (2010).

4.1.1. El Carchi ProvinceIn this province, Aguas Hediondas spring (P1 in Fig. 3) has a fairly

high temperature (50 °C) and a relatively low concentration of As(0.020 mg L−1). The Aguas Negras springs (P5) As concentration isthe lowest of the province (0.002 mg L−1). In Rumichaca and LaCalera springs (P8 and P12 in Fig. 3), the As concentrations are highcompared with other thermal waters, reaching 0.403 mg L−1 and0.684 mg L−1, respectively. The average extractable As for the sedi-ments at Aguas Hediondas spring (P1) increases downstream from170.7 to 717.6 mg kg−1. Two main factors contribute to the build-up of As in sediments: i) co-precipitation with calcite or calcium sul-fate since the geothermal waters are oversaturated with respect tothese minerals and ii) sorption on Fe(III) and Al(III) oxides and natu-ral organic matter. Chemical analysis of sediment samples confirms

Fig. 4. Study area in the north-central Andean region of Ecuador and

the presence of sulfur, iron, aluminum, and calcium. Thus, arsenic isretained within calcite and gypsum or is bound to large surfaceareas of oxyhydroxides or hydrated Fe(III) oxides (HFO) particles.

4.1.2. Imbabura ProvinceThe Chachimbiro geothermal springs have As concentrations of up

to 0.976 mg L−1 (0.876 mg L−1 on average). Environmental condi-tions contributing to elevated concentrations of As in Chachimbirogeothermal water include high bicarbonate concentration in thesprings (bicarbonate=216 mg L−1 in average), triggering the forma-tion of soluble arsenite–carbonate complexes such as As(CO3)2−,As(CO3)(OH)2−, and AsCO3

+ that hinder sorption of As on sediments(Kim et al., 2000). These anions can compete with arsenates for sorp-tion sites on metallic oxide surfaces thus keeping As in aqueousphase. Indeed, As content is not high in sediments of Chachimbirosprings (P17 and P16, 131.9 and 176.7 mg kg−1, respectively) despitethe high concentration in the waters.

4.1.3. Pichincha ProvinceSix geothermal water localities were analyzed for arsenic levels. Ara-

uco springs show As concentrations below the detection limit of0.002 mg L−1 (P22 in Fig. 3) and Cununyacu, La Merced de Nono, andIlalo exhibit concentrations above 0.200 mg L−1 (P25, P28, P35). ElTingo (P33) and La Merced (P34) springs, which are also part of theIlalo geothermal system, have As concentrations of 0.1 mg L−1, and tem-peratures of 42.6 and 36 °C, respectively. Analysis of sediments showhighly extractable As at La Merced de Nono (P28: 329.7 mg kg−1). HighCO2 concentrations at this site (640 mg L−1) may prevent even greaterbuild-up of As in sediments due to the formation of As-bicarbonate com-plexes (Sahai et al., 2007).

4.1.4. Cotopaxi ProvinceResults from the groundwater springs of Altamira farm show As

concentrations in the range of 0.012 to 0.047 mg L−1 (P38, P39). Incontrast, the concentration of As in sediments is relatively high(230 mg kg−1 in average). Additionally, organic matter content is

location of sampling sites. Modified from Cumbal et al. (2009).

Fig. 5. Study area covering Papallacta and Sucus Lakes and the Tambo and Sucus Rivers. Sampling sites at the Tambo River are also shown (modified from Cumbal et al. (2009)).

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around 29.5 wt.% in sediment samples, favoring the formation of As-organic matter associations in the sediments.

4.1.5. Tungurahua ProvinceHigh As concentrations are observed in the springs of Agua Santa

(P45, As=0.114 mg L−1) and of El Salado (P47, As=0.048 mg L−1), lo-cated in thenorthern slopes of the Tungurahua volcano. Temperatures inthese two springs are 54 and 48.9 °C, respectively. At Cunungyacu hotspring (P48), the As concentration is 0.047 mg L−1 with a pH of 8.3and temperature of 42.6 °C. Arsenic concentration in sediments of El Sa-lado geothermal spring (P47) is 198.7 mg kg−1. Ferric oxide concentra-tion in sediments is 128 mg Fe g−1 of sediment, with sorption onto thesolid phase as the probable main mechanism of As accumulation.

4.2. Papallacta lake basin

Papallacta Lake is located in Quijos County, Napo Province, in thenortheastern part of the country at an average altitude of 3360 m a.s.l..The lake formed as a result of a lava flow, known as Antisanilla, blockinga stream in 1760 (Bourdon et al., 2002), withwater covering an averagearea of 330,000 m2. The lake receives water from the Tambo River,some small cold streams, uncontrolled residual discharges from theJamanco hot springs, and thermal groundwaters on the north side ofthe lake (Cumbal et al., 2009). Small thermal streams discharge intothe Tambo River along approximately 12.8 km, impacting the waterquality of the river and lake (see Fig. 5).

4.2.1. Arsenic in geothermal springs along the Tambo RiverArsenic levels in geothermalwaters in the Papallacta basin are in the

range of 1.090–7.852 mg L−1. Speciation of As in two hot springs (GS1and GS7 in Fig. 5) indicates the dominance of As(III) which amounts to74.4 and 61.2% of total As concentration in highly reducing conditions.Total As concentrations for these springs are 3.152 and6.120 mg L−1, re-spectively. In addition, iron precipitates found in these geothermalsprings are abundant, suggesting that the input waters are rich in Fe.Reductive dissolution of iron and manganese minerals, as observed inBangladesh (Ahmed et al., 2004; Smedley and Kinniburgh, 2002) couldbe the main mechanism of As release during the uprising of reducinggeothermal waters. Iron oxidation and precipitation of iron hydroxidesoccur as the geothermal waters reach the atmosphere. As(V) is predom-inant in springs GS3 (67.8%) and GS6 (66.5%), whose total As

concentrations are 3.555 and 7.852 mg L−1, respectively. Temperaturesof springs GS3 andGS6 are low (15.6 and 13.8 °C), probably the result ofthe mixing of spring waters with shallow groundwaters or oxygenatedwater recharge.

4.2.2. Arsenic in Papallacta LakeSeven streams with water flows ranging from 0.3 to 4.5 L s−1

(Tambo River — 220 to 1508 L s−1) and unmeasured widespread ther-mal groundwaters discharge into Papallacta Lake. The highest As con-centration comes from the Tambo River (0.149 mg L−1) while all otherstreams exhibit concentrations in the range of 0.002 to 0.013 mg L−1.Samples of water taken on the surface of the lake showed variations inAs concentration from 0.22 to 1.74 mg L−1 on April 22, 2006 and resultsfrom July 20, 2006 showed smaller values ranging from 0.086 to1.43 mg L−1. This variability in As concentration is mainly due to sea-sonal circulation patterns.

Sediment samples taken on the eastern and southeastern shorescontain relatively high arsenic (540 and 613 mg kg−1) in contrast tothe lower sediment As levels on the northwestern and southwesternmargins (60 and 72 mg kg−1). This difference may be due to the con-tinuous removal and re-distribution of sediments caused by theTambo River input and also to As leaching from mineral and organicfractions of these sediments.

5. Bolivian Altiplano

Taquichiri et al. (2005) described the uses, location, and generalchemistry of thermal springs in Bolivia, specifically the Bolivian Altipla-no (BA), whose topographic elevation ranges from 3600 to 3900 m a.s.l.(PPO-3, 1996). The BA is an intra-mountain basin enclosed by thewest-ern and eastern chains of the Andes. Two systems presenting high Asconcentrations in the BA are considered here: the Poopo Lake basinand the salars of Coipasa and Uyuni.

5.1. Chemical Composition of the Poopo Lake basin Thermal springs

The Poopó Lake basin in the Oruro area is located in the central BA atan altitude of 3686 m a.s.l. Poopó Lake has an area that varies from2650 km2 to 4200 km2 on a seasonal basis (Quintanilla, 1994). The ther-mal springs (Fig. 6) are commonly used for consumption, irrigation, andrecreational purposes. The spring pH values range from 6.3 to 8.3 with

68 D.L. López et al. / Science of the Total Environment 429 (2012) 57–75

an average of 7.0 and temperatures range from 40 to 75 °C. Water sam-ples indicate a diversity of water types: 50% are Na–Cl-bicarbonate,43.7% are Na–Cl, and 6.2% are Na-bicarbonate (see SupplementalTable). Dissolved As concentration in the thermal springs ranges from0.008 to 0.065 mg L−1 and average 0.023 mg L−1 (n=16). The highestconcentration of As is present in the Na–Cl water type (Ormachea etal., 2010). Arsenic speciation indicates that the predominant species isAs(III) in nine samples and As(V) in five samples. Arsenic in the thermalsprings could be attributed to the oxidation of sulfide minerals such asarsenopyrite and to the dissolution of volcanic rocks.

5.2. Salars of Coipasa and Uyuni

Surface water drainage were studied in the catchment areas of thesalars of Coipasa and Uyuni located at the Bolivian Altiplano south-west of the Poopó Lake catchment area by Banks et al. (2004).Concentrations of As were up to 4600 mg L−1 with a median of0.034 mg L−1 and were attributed not only to geothermal springs andfumarole sulfur deposits, but also to the oxidation of sulfidic minerals.Values of TDS increase from headwaters to downstream as a

50 Km

ORURO CIT

URU URULAKE

SOUTHAMERICA

LEGEND Main City Villages Road Sampling Sites 1 Soracachi 2 Obrajes 3 Capachos 4 Machacamarca 5 Poopó 6 Cabrería 7 Pazña 8 Urmiri 1 9 Urmiri 3 10 Urmiri 4 11Malliri 12 Phutina 13 Aguas Calientes 14 Challapata 15 Castilluma 16 Vichailope

PaHu

Fig. 6. Map of the BA showing the basins of Lake Titicaca, Desaguadero River, Uru Uru andsurface and groundwater monitoring. The study area and sampling sites of thermal springs

consequence of evaporation processes and dissolution of evaporites.In comparison, concentrations of dissolved As decrease slightly down-stream, probably as a consequence of adsorption on ferric oxyhydrox-ides of stream sediments.

6. Northern Chile

Two different systems with high As concentrations are consideredfrom northern Chile: (1) the Camarones River area in the Arica andParinacota regions (Fig. 7) and (2) the Tatio geothermal field.

6.1. Camarones River

The origin of As in the surface waters of the Arica and Parinacota re-gions (Fig. 7) is traced to the volcanic activity of the Andean Cordilleraand As minerals contained in ignimbrites and other volcanic rocks aswell as in sulfur calcretes (Mansilla and Cornejo, 2002). The CamaronesRiver in this area has a high average concentration of As (1.0 mg L−1)(Cornejo et al., 2006a, 2006b, 2008; Lara et al., 2006; Yañez et al.,2005), forming from the confluence of two tributaries, the Ajatama

10

12

14

15

16

13

11

1 2

3

4

5 6

7

8 9

Y

POOPÓ LAKE

BOLIVIA

mpa llagas

Quillacas

Machacamarca

Challapata

Huari

Poopó

Pazña

StudyArea

Poopó lakes, and the Coipasa and Uyuni salars. Numbers indicate sampling stations forare located southeast of the city of Oruro (modified from Quintanilla et al., 2009).

Fig. 7. Sites in northern Chile with high As concentrations from geothermal sources: A) Camarones River watershed and B) El Tatio geothermal field.

69D.L. López et al. / Science of the Total Environment 429 (2012) 57–75

and Caritava Rivers (Fig. 7). Thewaters of the AjatamaRiver have lowAsconcentrations (lower than 0.001 mg L−1). In contrast, the CaritayaRiver has an As concentration of 2.35 mg L−1 (Fig. 7). The As enrich-ment of the Caritaya River is likely due to two different processes: in-puts from three As-rich lakes and leaching of As-rich soils and rocks.

For the first process, high As concentrations are discharged into theriver from the region of Amuyo Lakes (Fig. 7), which receive hydrother-mal discharges at an elevation of 3700 m a.s.l., higher than the CaritayaRiver. Three lakes in the area have distinct colors that provide theiridentification: Red Lake (Wilacota), Green Lake, and Yellow Lake. Thewaters of these lakes contain the highest levels of As, B, and dissolvedsolids, with a mean As concentration of 11.0 mg L−1.

For the second process, precipitation in the high Cordillera region isgenerally accompanied by electrical discharges from lightning that pro-mote the formation of NOx type gases responsible for acid precipitation(Mansilla and Cornejo, 2002). Infiltration of the acidic water and its re-action with As minerals increases the As concentration (Mansilla andCornejo, 2002). This process explains the occurrence of rivers withhigh concentrations of As, such as the Camarones, Vitor, and Lluta Riversin the transverse valleys of the region (Bundschuh et al., 2008). Arsenicconcentration in these rivers decreases toward the ocean (Fig. 7). Thepresence of iron minerals in river sediments is probably producingsorption reactions that decrease the concentration of As in the oxidizingconditions that prevail along the river.

6.2. El Tatio geothermal system

El Tatio geothermal system is located in the Antofagasta region ofnorthern Chile (Region II, Fig. 7) at an elevation of 4200 m a.s.l., and100 km east of the town of Calama. It is the largest geothermal field inthe southern hemisphere (Fernandez-Turiel et al., 2005), and a signifi-cant source of As and other typical geothermal elements such as F, Sb,Li, and B. Dozens of thermal springs, fumaroles, geysers, and boilingandmudpools occur in fourmain areas: Central,West, Corfo, andGeyserBlanco. The main hydrothermal reservoir is confined within the perme-able Puripicar Formation and its Salado Member and recharge takesplace about 15 km east of the field (Giggenbach, 1978; Tassi et al.,2010). The discharging water has a temperature of about 86 °C and isof Na–Cl type. Arsenic concentration in the waters of one geyser is7.6 mg L−1, in two hot springs it is 27.0 and 30.1 mg L−1, and in the Sa-lado River it is 21.0 mg L−1. Deuterium and oxygen isotopes from ElTatio waters show that they are the result of mixing between andesite-influenced water and meteoric precipitation (Tassi et al., 2010).

High dissolved silica concentrations at El Tatio result in a massiveprecipitation of siliceous sinter, mostly composed of amorphous silica,in discharge channels, (Fernandez-Turiel et al., 2005). The structure ofthe sinter depends on temperature and type of bacterial communities(Landrum et al., 2009). Arsenic is not incorporated into siliceous sinterand is quitemobile with almost constant As:Cl ratios several kilometers

SO42-

AlkalinityCl-

Fig. 8. Triangular diagram for SO42−, Cl, and alkalinity of geothermal waters of Latin

America. Concentrations of As are as follows: small circles=Asb0.050 mg L−1, smallopen triangles=0.050bAsb1.00 mg L−1, open small squares=1.00bAsb10.0 mg L−1,and large dark squares=10.0bAs mg L−1. Note high As concentrations for the watersthat are rich in Cl.

70 D.L. López et al. / Science of the Total Environment 429 (2012) 57–75

downstream (Landrum et al., 2009). This is caused by extremely highaqueous silica concentrations that cover the surfaces of ferric oxyhydr-oxides with siliceous phases, inhibiting the adsorption of As, especiallyof As(III)(Swedlund andWebster, 1999). In contrast to As, Sb partitionsinto siliceous geyserite, probably co-precipitating as the nanoparticu-late mineral cervantite during cooling (Landrum et al., 2009) and be-coming quite immobile in comparison with As.

Discharge of geothermal arsenic in El Tatio geothermal field has astrong impact on the Rio Loa, which is used for the water supply ofAntofagasta city (Romero et al., 2003). The As input occurs via the Sa-lado River, which is a tributary of the Rio Loa, and the As is sourcedfrom El Tatio geothermal waters. Arsenic concentrations in the riversediments are up to 11,000 mg kg−1 close to El Tatio and remainaround 700 mg.kg−1 at the mouth of Rio Loa. Dissolved As concentra-tions in the Salado River decrease downstream due to dilution orsorption reactions, but they still are as high as 1.2 mg L−1 at the con-fluence with the upper Rio Loa.

7. Discussion

The presence of As in geothermal systems, mainly in geothermalwaters and their impact on low-temperature aquifers, surface waters,and other surface environments are important in many geologic set-tings in Latin America. Geothermal As may degrade drinking waterresources, as shown by the examples of Rio Loa (Chile), PapalactaLake (Ecuador), and Ilopango and Coatepeque lakes (El Salvador).

Arsenic is leached from the host rocks of the geothermal reservoir,where high residence time, temperature and pressure, and reducing con-ditions (presence of As(III)), favor the release of As from rock minerals(Webster and Nordstrom, 2003). This leaching occurs along with otherelements, such as Sb, B, F, Li, Hg, Se, Tl, and hydrogen sulfide (H2S), mak-ing them good indicators of mixing of geothermal waters with low tem-perature aquifers or surface waters. Table 1 gives an overview of Asconcentrations, temperature, salinity, and pH in fluids of geothermalwells and geothermal springs in Latin America. A complete chemicaldata set is presented in the Supplemental Table. Generally, the highestAs concentrations, typically from mg L−1 to tens of mg L−1, are foundin the fluids of geothermal reservoirs in volcanic rocks, and can be ashigh as 30.1 mg L−1 in El Tatio and 73.6 mg L−1 at Los Humeros, Mexico.Lower As concentrations are found in the hot springs of Guatemala, ElSalvador, and Honduras, where As contents in fluids discharging fromvolcanic hydrothermal systems are as low as 0.010 mg L−1, which canbe explained bymixingwith shallower groundwater, or lower residencetime in the reservoir rocks. Comparedwith geothermal reservoirs hostedin volcanic rocks, much lower fluid As concentrations are found in geo-thermal reservoirs located in sedimentary rocks such as at Cerro Prieto(México) with a range of 0.25 to 1.5 mg L−1 (Birkle and Bundschuh,2009; Birkle et al., 2010).

Geothermal waters change their chemical composition during theirascent from the geothermal reservoir to the ground surface due to sev-eral physical and chemical processes. Fluids, which ascendwithout lossof heat (or limited loss of heat due to conductive cooling), will emergeas Na–Cl waters with near neutral pH, high silica content, and aCl−/SO4

2−ratioN1. Fluids that correspond to this description are dis-charges from wells in geothermal fields under exploitation (e.g. LosAzufres in Mexico, Berlín in El Salvador, Zunil in Guatemala, seeTable 1). These Na–Cl waters generally show the highest As concentra-tions of several mg/L. Rising geothermal waters, whenmixed with nearsurface groundwater rich in bicarbonate become waters of bicarbonatetype (e.g., Platanares PL-1, Honduras). Waters with a high content ofH2S gas, which condense near the ground surface, form pools of waterwith high SO4

2− and lowCl− concentrations (e.g., hot spring F-83 in Ber-lín geothermal field, El Salvador). Oxidation of H2S in Na–Cl waters inthe near surface environment gives rise to low pH and high SO4

2− andCl waters (e.g. Albergue Agroecológico hot spring, Rincón de la Viejavolcano, Costa Rica, see Supplemental Table). Oxidized sulfur forms

bisulfate ions (HSO4−), a reaction favored by decreasing temperature

in the near-surface groundwater environment, producing low pHwater. For the geothermal waters of Latin America, the distribution ofall these types of waters are shown in the triangular plot for SO4

2−,Cl−, and alkalinity (HCO3

−+2CO32−) of Fig. 8. Themajority of thewaters

with As concentrations higher than 10mg L−1 are Cl-rich waters, and afew of them have bicarbonate as the main anion. Only four of the higharsenic waters have concentrations of sulfate comparable to Cl and bi-carbonate, three being lake waters from Amuyo Lake in northern Chile(Fig. 7). Relatively low concentrations of sulfate for As rich watershave been observed in other natural systems of the world, such as theglacial aquifers of central Illinois (Kelly et al., 2005) and in groundwa-ters from Bangladesh (Ravenscroft et al., 2001).

Precipitation of As due to bacterial reduction of sulfate has beenproposed as a mechanism to explain low sulfate in high As waters(Rittle et al., 1995). Ravenscroft et al. (2001) suggested that the lackof correlation between As and sulfate in Bangladesh groundwaters in-dicated that As was not coming from the oxidation of arsenopyrite.This lack of correlation is also observed in geothermal waters fromLatin America (Fig. 8). However, pyrite and arsenopyrite are commonminerals in geothermal reservoir rocks. The lack of correlation isprobably due to a reduction of the solubility of As minerals when sul-fur is present promoting precipitation rather than dissolution of Asminerals. Reducing conditions prevail along the pathways of ascend-ing geothermal water until near the Earth's surface zone, where at-mospheric O2 becomes increasingly available, either during mixingof the geothermal water with cold water of oxidizing aquifers orwhen it discharges at the ground surface. At the surface, exposureto oxygen results in a fast oxidation of As(III) to As(V) and precipita-tion of different mineral phases (Alsina et al. 2007, Bundschuh et al.,2008), which removes As from the fluids to a variable extent. Al-though accumulated sediments at the bottom of evaporation pondsat the Los Azufres geothermal field (Mexico) are composed of morethan 95% amorphous silica, the average abundance of 72.8 mg/kg forAs confirms the precipitation potential of this element under oxidiz-ing and/or cooling conditions (Birkle and Merkel, 2002).

Arsenic and Cl remain in the liquid phase during sub-surface boilingand phase separation (Webster and Nordstrom, 2003). Geothermal wa-ters rich in Cl are usually also rich in As. However, Birkle et al. (2010)found a lack of significant correlation between As and Cl for theMexicanvolcanic geothermal fluids. These authors postulate that As and Cl didnot have the same origin, and suggested rock leaching as the majorchemical process producing abundant As in Mexican volcanic geother-mal fluids. This lack of statistically significant correlation is also ob-served in the data reported in the Supplemental Table for the different

0

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0.0 1.0 100.0 10000.0

B (mg L-1)

As<0.05 mg L-1 0.05>As>1.0 mg L-1

1.0>As>10.0 mg L-1 As>10.0 mg L-1

Li (

mg

L-1

)

Fig. 10. Li vs. B concentrations in geothermal waters of Latin America. Arsenic concen-trations in Fig. 7. Note high concentrations of B and Li in waters with high concentra-tions of arsenic.

71D.L. López et al. / Science of the Total Environment 429 (2012) 57–75

geothermal sources of Latin America. Graphs showing the concentra-tion of K vs. Na and Li vs. B from Supplemental Table data are inFigs. 9 and 10. In thesefigures, As concentration is shown in four ranges:Asb0.050, 0.050bAsb1.0, 1.0bAsb10, and AsN10 mg L−1. In Fig. 9, apositive linear trend of Na vs. K shows that geothermal waters withmore than 10 mg L−1 of As have Na and K concentrations higher than87 and 18 mg L−1, respectively. In comparison, geothermal waterswith less than 0.050 mg L−1 As have Na and K lower than 73 and22 mg L−1. Similarly, in Fig. 10, geothermal waters with AsN10 mg L−1

have B and Li higher than 26 and 6 mg L−1, respectively, except in thewells of Los Humeros geothermal field in Mexico, where Li contentsare lower than 1 mg L−1 even with As concentrations higher than10 mg L−1 (see Supplemental Table). For these four ions, rock leachingis the dominant mechanism of their enrichment in water, suggestingthat for As, this mechanism also dominates. Depleted Li values in LosHumeros fluids are likely due to reservoir rocks with low Li content.The enrichment of Cl in geothermal fields may come from host rockleaching, seawater intrusion, evaporation of seawater prior to infiltra-tion, and/or gaseous HCl dissolution of magmatic components (Birkleet al., 2010).

Arsenic occurs predominantly in pyrite in volcanic rocks, or associat-ed with Fe oxide in geothermally altered rocks (Ballantyne and Moore,1988). Additionally, As-rich smectite (1500 to 4000 mg kg−1 As) hasbeen found in geothermal precipitates in NW Japan (Pascua et al.,2005). As primary As minerals have not been documented for any ofthe described geothermal reservoirs in Latin America, abundant clayminerals in addition to sulfide minerals, could be a potential sourcefor As dissolution during water–rock interaction under elevated tem-perature conditions (N280 °C). The absence of primary As minerals isa common feature for global geothermal host rocks (Ballantyne andMoore, 1988). As postulated by Webster and Nordstrom (2003) forglobal geothermal water, neither magmatic fluids input, nor As miner-alization, is a prerequisite for As enrichment in geothermal fluids.

The As dissolution process from host minerals is generally acceler-ated by elevated temperature conditions. Fluids from volcanic reser-voirs (e.g., Los Azufres and Los Humeros from the TMVB, Las TresVírgenes from a granodioritic basement, and the basaltic–andesiticreservoir of Berlín) show relatively constant As concentrationsthrough varying temperature conditions, which indicates that tem-peratures above 230 °C to 250 °C provide optimal and stable condi-tions for As dissolution, in addition to high Cl concentrations (Birkleet al., 2010). In contrast, temperature conditions for sedimentary

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1 10 100 1000 10000 100000

Na (mg L-1)

As<0.05 mg L-1 0.05>As>1.0 mg L-1

1.0>As>10.0 mg L-1 As>10.0 mg L-1

K (

mg

L-1

)

Fig. 9. K vs. Na concentrations in geothermal waters of Latin America. Arsenic concen-trations in Fig. 7. Note high concentrations of Na and K in waters with high concentra-tions of arsenic.

reservoirs are not important for water–rock interaction processes, assuggested by low As concentrations for Cerro Prieto geothermal fluids(Birkle et al., 2010).

Volcanic gas condensates usually show As concentrations lower than0.5 mg L−1 (e.g., Colima, Cerro Negro, Momotombo), consistent withtheir high sulfur species concentration. Data in Table 1 shows; watersinfluenced by active volcanoes have much lower As concentrationsthan thosemeasured in recently exploited geothermal reservoirs. For ex-ample, at Colima volcano magnesium normalized enrichment factorsand gas geochemistry (Taran et al., 2001) indicate As ascent from shal-low degassing magma. At Popocatépetl, the presence of As in ash leach-ates and of tennantite that crystallized from magmatic volatiles inpumice samples (Larocque et al., 2008), provide evidence for its trans-port in gaseous emissions. However, concentration increases in thehigher pH and most mineralized springs, indicating As release from thehost rock, as well as possible concentration increase due to evaporation.

Calcium and magnesium concentrations in geothermal waterscould also reflect the rock leaching process. However, Ca vs. Mg con-centrations does not show a significant correlation. Carbonates, suchas calcite, as well as clay minerals, are characteristic of geothermalmineral alteration (Giggenbach, 1988) and can buffer the composi-tion of the solution. The dissolution of calcite is as follows:

CaCO3 þ Hþ ¼ Ca

2þ þ HCO−3

with an equilibrium constant K=aCa2þaHCO3

−

aHþ, enables Ca2+/H+ to be

plotted vs. HCO3− concentration for the studied geothermal waters,

as presented in Fig. 11. The bicarbonate concentration is calculatedusing pH and alkalinity values and the second dissociation constantfor carbonic acid at the reported temperature. The equilibrium linesfor calcite dissolution at 25 °C and 100 °C and the second dissociationconstant for carbonic acid as a function of temperature were obtainedusing values reported in Shiraki and Brantley (1995). The majority ofthe data fall in a cluster around the 25 °C and 100 °C equilibrium linessuggesting that the solutions are close to or at equilibrium with calci-um carbonate at the discharge temperatures.

Determination of As(III) has been reported only for three samplesfrom Ecuador. However, the relatively low sulfate concentrations incomparison to bicarbonate and Cl (Fig. 8) suggest that As(III) is likelythe dominant oxidation state for As in deep geothermal waters. When

1.0E-02

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As>10.0 mg L-1 Equilibrium 25 C Equilibrium 100 C

Calcite Saturation

Calcite Undersaturation

Ca2+

/H+

Fig. 11. Ca2+/H+ vs. HCO3− concentrations in geothermal waters of Latin America. Ar-

senic concentrations in Fig. 7. Data are clustered close to equilibrium conditions withrespect to calcite.

72 D.L. López et al. / Science of the Total Environment 429 (2012) 57–75

reaching the ground surface, arsenite is quickly oxidized to As(V) (e.g.,Papallacta basin in Ecuador). When geothermal water discharges intoshallow aquifers, or surface waters, significant amounts of As will re-main in solution, either as As(V) or as As(III). Arsenic species distribu-tion depends on residence time of the groundwater in the aquifer andon the kinetics of As(III) oxidation.

A significant As content can be expected only in those geothermalwaters which contain a significant component of geothermal reservoirwater, which corresponds generally to Na–Cl waters. This is in contrastto shallow groundwater, which is heated by conduction or steam as ob-served in Rincón de La Vieja field, Costa Rica, where only two from 30geothermal springs exhibited As concentrations of several mg L−1, cor-responding to those of the deep geothermal reservoir. Springs with ele-vated As concentrations (up to several 100 μg L−1) indicate that thespring water has a subordinate component of reservoir water, asshown for springs in El Zunil (Guatemala) and Berlín (El Salvador).

After the discharge of geothermal waters at the ground surface, sev-eral As attenuation processes take place in surfacewater bodies. Arsenicconcentrations are diluted bymixingwith As-free surface streams. Arse-nic is also generally adsorbed on the surface of precipitated ferric oxy-hydroxides. However, high concentrations of dissolved silica, whichare common in discharging geothermal waters, may inhibit As adsorp-tion due to saturation of the surface of ferric iron minerals as observedat the El Tatio site (Landrum et al., 2009). Furthermore, evaporative en-richment canmaintain highAs concentrations inwaters in spite of As re-moval by adsorption on stream sediments. This has been observed inregions with arid and semi-arid climate such as the Rio Loa catchmentin the AtacamaDesert of Chile (Romero et al., 2003) and around the sal-ars of Coipasa and Uyuni of the Andean Altiplano in Bolivia (Banks et al.,2004). Evaporation also increases pH and Cl concentrations and, thus,contributes to desorption and high mobility of As, which generallyis present as oxyanionic species in surface streams (Banks et al.,2004; Sracek et al., 2004).

8. Conclusions

Data on As concentration in volcanic fluids and geothermal systemsat several sites in Latin America suggest that there are two differentsources of As in volcanic regions. One source is linked to volcanicgases emitted from volcanic craters after partitioning of As betweenthe gas phase and the magma body, and the other is the As contained

in the erupted volcanic products that form the volcanic edifice. Leachingof As from the volcanic rocks occurs when groundwater with dissolvedvolcanic gases circulates throughout the volcanic edifice. Results for theAs composition of volcanic gases are scarce and the data available give avery wide range of concentrations, from 0.050 to 1.2 mg L−1 for Mexi-can volcanoes, to 0.013 to 14.7 mg L−1 and 0.031 to 0.40 mg L−1 forCosta Rican and Nicaraguan volcanoes, respectively. The variability ofthe data even for a single volcano (e.g., see Póas Volcano above) canbe explained by multiple factors. One is the difference in sampling andlaboratory procedures that diverse researchers have used, and anotheris the state of activity of the volcano. In the same way that concentra-tions of carbon dioxide, sulfur dioxide, and other gases change withthe state of activity of a volcano (e.g. Giggenbach, 1988; Todesco et al.,2004), it is probable that As concentrations also fluctuate. In addition,a clear understanding of the way of how As partitions between gasand liquid phases within a magmatic chamber is not possible yet. Fur-ther experimental, field, and theoretical research is needed to under-stand these partitioning processes at high temperatures.

Data for the geothermal systems presented in this paper aswell as forother geothermal systems of the world (Planer-Friedrich et al., 2006)suggest that Na–Cl mature waters of volcanic systems present the high-est As concentrations. Na–Cl waters typically have relatively low sulfate,high salinity, highNa andCl concentrations, and a pHusually higher than8. High As concentrations are commonly combined with elevated Li andB contents. Na–Cl waters are the result of a long-termwater–rock inter-action between groundwater enriched in volcanic gases andheated closeto themagmatic chamber and the host rock that these waters encounteralong their flowpath (Giggenbach, 1988). They are usually discharged atthe base or flanks of volcanoes and represent the main water source formany geothermal fields in exploitation.

As indicated by Birkle et al. (2010), the Na–Cl geothermal water isaffected by ebullition during its ascent due to changes in hydrostaticpressure and by condensates at shallower levels forming watersthat are low in As as observed in the surficial discharges of severalgeothermal systems in Latin America (e.g., Los Humeros geothermalfield, Berlín geothermal field). Mixing of Na–Cl geothermal waterswith shallower meteoric waters can produce bicarbonate-rich waterswith low to intermediate As concentrations (e.g., Momotombo geo-thermal field).

Data for the studied sites in Ecuador, Northern Chile, and IlopangoLake indicate that the fate of As after water discharge from thermalsprings depends on the chemical composition of the water and sedi-ments (e.g., presence of ferric iron minerals able to sorb the As) andthe concentration of organic matter. Oxidation of As(III) to As(V) occursas thewaters come into contactwith the atmosphere. The oxidation pro-cess seems to be catalyzed in the presence of organic matter by bacteriaattached to macrophytes (Wilkie and Hering, 1998). The presence of bi-carbonate, chloride, and silica in solution seems to inhibit the sorptionprocess of As by iron minerals, as observed at La Merced de Nono in Ec-uador. Immobilization of As by sorption onto iron minerals in surfacewaters is an important natural remediation process for As contamina-tion. The data reported in this paper (different provinces in Ecuador,Camarones River in Chile, outlet river from Ilopango Lake in El Salvador)suggest that natural remediation of As contamination is occurring, andexplains why As contamination is more important in groundwatersthan in surfacewaters. For the case of caldera lakes rich in As (as in Coat-epeque and Ilopango lakes of El Salvador) or for other lakes such asPapallacta and Amuyo Lakes in Ecuador and Chile, respectively, thewater/sediment ratio is high and does not allow for natural attenuation.Significant transfer of As fromwater to sediments can occurwhenwatercirculates along rivers where the water/sediment ratio is lower than inlakes and conditions are more oxygenated, favoring sorption processesin iron minerals. Other processes such as evaporation can cause in-creases in pH and can maintain high As concentrations in waters inspite of As removal by sorption on stream sediments, as observed inarid and semi-arid climate regions (e.g., Camarones River).

73D.L. López et al. / Science of the Total Environment 429 (2012) 57–75

More research on As in volcanic and geothermal systems is neededfor a better understanding of the processes that determine the con-centration of As in groundwater, volcanic gases, and surface waters.More extensive monitoring of the water quality and health of the in-habitants of the Latin American Pacific Rim is crucial for amelioratingthe effects of arsenic in waters.

Supplementary material related to this article can be found onlineat doi:10.1016/j.scitotenv.2011.08.043.

Acknowledgments

The authors want to acknowledge the support of different organiza-tions for portions of this work: Fundación Amigos del Lago de Ilopango,Swedish International Development Cooperation Agency (Sida Contri-bution: 7500707606), Swedish International Development CooperationAgency project to Costa Rica on Arsenic in Geothermal Waters, CentralAmerica (SWE-2005-P18), Convenio de Desempeño Universidad deTarapacá (UTA-MINEDUC, 2008–2010), and the CYTED Proyect RedTemática 406RT0282 Iberoarsen. We thank Dr. John Farmer and Dr.Elizabeth Gierlowski-Kordesch for editorial help.

References

Acharya H. Influence of plate tectonics on the locations of geothermal fields. Pure ApplGeophys 1983;212:853–67.

Adams A, Dennis B, Van Eeckhout E, Goff F, Lawton R, Trujillo PE. Results of investiga-tions at the Zunil geothermal field, Guatemala: well logging and brine geochemis-try. Los Alamos National Laboratories Report LA-12098-MS; 1991.

Ahmed KM, Bhattacharya P, Hasan MA, Akhter SH, Alam SMM, Bhuyian MAH. Arsenicenrichment in groundwater of the alluvial aquifers in Bangladesh: an overview.Appl Geochem 2004;19:181–200.

Agostini S, Corti G, Doglioni C, Carminati E, Innocenti F, Tonarini S, et al. Tectonic andmagmatic evolution of the active volcanic front in El Salvador: insight into the Ber-lín and Ahuachapán geothermal areas. Geothermics 2006;35:368–408.

AldrichMJ, Eppler D, Heiken G, FloresW, Ramos N, Ritchie A. San Ignacio (La Tembladera)geotermal site, departamento de Francisco Morazán, Honduras, Central America. LosAlamos National Laborator Report LA-10696-MS; 1987.

Armienta MA, De la Cruz-Reyna S. Some hydro-geochemical fluctuations observed inMéxico related to volcanic activity. Appl Geochem 1995;10:215–27.

Armienta MA, De la Cruz-Reyna S, Macías JL. Chemical characteristics of the crater lakesof Popocatépetl, El Chichón and Nevado de Toluca volcanoes, México. J VolcanolGeotherm Res 2000;97:105–25.

Armienta MA, De la Cruz-Reyna S, Morton O, Cruz O, Ceniceros N. Chemical variationsof tephra deposit leachates for three eruptions from Popocatépetl volcano. J Volca-nol Geotherm Res 2002;113:61–80.

Armienta MA. Laboratorio de Química Analítica Internal Reports. UNAM, México, D.F:Instituto de Geofísica; 2008.

Armienta MA, Vilaclara G, De la Cruz-Reyna S, Ramos S, Ceniceros N, Cruz O, et al.Water chemistry of lakes related to active and inactive Mexican volcanoes. J Volca-nol Geotherm Res 2008a;178:249–58.

Armienta MA, De la Cruz-Reyna S, Gómez A, Ramos E, Ceniceros N, Cruz O, et al. Hydro-geochemical indicators of the Popocatépetl volcano activity. J Volcanol GeothermRes 2008b;170:35–50.

Arellano VM, Izquierdo G, Aragón A, Barragán RM, García A, Pizano A. Initial pressuredistribution at the Los Humeros geothermal field, Puebla, Mexico. Ing HidraulMex 2001;XVI:75–84.

Arellano VM, García A, Barragán RM, Izquierdo G, Aragón A, Nieva D. An updated con-ceptual model of the Los Humeros geothermal reservoir (Mexico). J VolcanolGeotherm Res 2003;124:67–88.

Banks D, Markland H, Smith PV, Mendez C, Rodriguez J, Huerta A, et al. Distribution,salinity and pH dependence of elements in surface waters of the catchmentareas of the Salars of Coipasa and Uyuni, Bolvian Altiplano. J Geochem Explor2004;84:141–66.

Baldock JW. Geología del Ecuador: Boletín Explicativo del Mapa Geológico de la Repúblicadel Ecuador. Quito: Dirección General de Geología y Minas; 1983.

Ballantyne JM, Moore JN. Arsenic geochemistry in geothermal systems. Geochim Cos-mochim Acta 1988;52:475–83.

Battocletti L, (Bob Lawrence & Associates, Inc.). Geothermal resources in Latin America& the Caribbean. Report for Sandia National Laboratories & the U.S. Department ofEnergy, Office of Geothermal Technologies, Contract No. As-0989; 1999.

Bennati L, Finizola A, Walker JA, Lopez DL, Higuiera C, Schütze C, et al. Fluid circulationin a complex volcano-tectonic setting, inferred from self-potential and soil CO2 fluxsurveys: the Santa María – Cerro Quemado – Zunil volcanoes and Xela caldera(northwestern Guatemala). J Volcanol Geotherm Res 2011;199:216–29.

Birkle P. Origen and environmental impact of Los Azufres geothermal water (original inGerman). Wiss. MitteilungenGermany: Institute for Geology, Technical UniversityFreiberg; 1998.

Birkle P, Merkel B. Environmental impact by spill of geothermal fluids at the geothermalfield of Los Azufres, Michoacán, México. Water Air Soil Pollut 2000;124:371–410.

Birkle P, Merkel B, Portugal E, Torres I. The origin of reservoir fluids at the geothermalfield of Los Azufres, México — isotopic and hydrological indications. Appl Geochem2001;16:1595–610.

Birkle P, Merkel B. Mineralogical–chemical composition and environmental risk poten-tial of pond sediments at the geothermal field of Los Azufres, Mexico. Environ Geol2002;41:583–92.

Birkle P. Development of geothermal energy in Mexico and its energetic potential forthe future. In: Klapp J, Cervantes-Cota JL, Chavez-Alcala JF, editors. Towards aCleaner Planet — Energy for the Future. Springer Verlag; 2007. p. 343–63.

Birkle P, Bundschuh J. High and low enthalpy geothermal resources and potentials. In:Bundschuh J, Alvarado G, editors. Central America — Geology, Resources and Haz-ards. The Netherlands: Taylor & Francis/Balkema Publishers; 2007a. p. 705–76.

Birkle P, Bundschuh J. Hydrogeochemical and isotope composition of geothermal fluids. In:Bundschuh J, Alvarado G, editors. Central America— Geology, Resources and Hazards.The Netherlands: Taylor & Francis/Balkema Publishers; 2007b. p. 777–838.

Birkle P, Bundschuh J. The abundance of natural arsenic in deep thermal fluids of geo-thermal and petroleum reservoirs in Mexico. In: Bundschuh J, Armienta MA, BirkleP, Bhattacharya P, Matschullat J, Mukherjee AB, editors. Natural arsenic in ground-waters of Latin America — Occurrence, health impact and remediation. Taylor &Francis Group; 2009. p. 145–53.

Birkle P, Bundschuh J, Sracek O. Mechanisms of arsenic enrichment in geothermal and pe-troleum reservoirsfluids inMexico.Water Res 2010. doi:10.1016/j.watres.2010.05.046.

Bourdon E, Eissen JP, Monzier M, Robin C, Martin H, Cotton J, et al. Adakite-like lavasfrom Antisana Volcano (Ecuador): evidence for slab melt metasomatism beneathAndean northern volcanic zone. J Petrol 2002;43:199–217.

Bundschuh J, Nicolli HB, Blanco MC, Blarasin M, Farías SS, Cumbal L. Distribución dearsénico en la región sudamericana. In: Bundschuh J, Pérez Carrera A, Litter M, ed-itors. IBEROARSEN Distribución del arsénico en las regiones Ibérica e Iberoameri-cana. Argentina: CYTED; 2008. p. 137–68.

Canul RF, Rocha VL. Informe geológico de la zona geotérmica de “El Chichonal”. Chia-pas. Morelia, México: Com. Fed. de Electr. (CFE); 1981. Informe 32–81.

Casadevall TJ, De la Cruz-Reyna S, Rose Jr WI, Bagley S, Finnegan DL, Zoller WH. CraterLake and ost-eruption hydrothermal activity, El Chichón Volcano, México. J Volca-nol Geotherm Res 1984;23:169–91.

Cedillo F. Hydrogeological model for the Los Humeros geothermal reservoir, Puebla,Mexico. Geotermia 1999;15:159–70.

Chandrasekharam D, Bundschuh J. Geochemistry of thermal waters and thermal gases.In: Chandrasekharam D, Bundschuh J, editors. Geothermal Energy Resources forDeveloping Countries. Swets & Zeitlinger; 2002. p. 253–67.

Clarke FW. The data of geochemistry. USUS Bull 1911;491:189.Connor CB, Clement BM, Song X, Lane SB, West-Thomas J. Continuous monitoring of

high-temperature fumaroles on an active lava dome, Volcán de Colima, Mexico —

evidence of mass flow variation in response to atmospheric forcing. J GeophysRes 1993;98:19713–22.

Cornejo L, Lienqueo H, Arenas M, Acarapi J, Yañez J, Mansilla H. In field arsenic removalfrom natural water by zero valent iron. Environ Pollut 2008;156:827–31.

Cornejo L, Lienqueo H, Arenas M, Acarapi J, Mansilla H, Yañez J. Remoción de arsénico deaguas naturales del río de Camarones y su utilización para consumo humano, med-iante la tecnología SORAS Modificada-Hierro Cero. In: Litter MI, editor. Resultados fi-nales del Proyecto OEA/AE 141: Investigación desarrollo, validación y aplicación detecnologías solares para la potabilización de aguas de zonas rurales aislados de Amér-ica Latina y el Caribe. La Plata Argentina: Digital Grafic; 2006a. p. 139–58.

Cornejo L, Lienqueo H, Arenas M, Acarapi J, Mansilla H, Yañez J. Transferencia de lametodología SORAS Modificada # Hierro Cero a los poblados que carecen deagua potable de la Comuna de Camarones, Arica, Chile. In: Litter Marta I, editor.Resultados finales del Proyecto OEA/AE 141: Investigación desarrollo, validacióny aplicación de tecnologías solares para la potabilización de aguas de zonas ruralesaislados de América Latina y el Caribe. La Plata, Argentina: Digital Grafic; 2006b.p. 159–98.

Cumbal L, Bundschuh J, Aguirre V, Murgueitio E, Tipán I, Chavez C. The origin of arsenicin waters and sediments from Papallacta Lake in Ecuador. In: Bundschuh J,Armienta MA, Birkle P, Bhattacharya P, Matschullat J, Mukherjee AB, editors. Natu-ral arsenic in groundwaters of Latin America — Occurrence, health impact and re-mediation. Taylor & Francis Group; 2009. p. 81–90.

Cumbal L, Vallejo P, Rodriguez B, López D. Arsenic in geothermal sources at the North-Central Andean Region of Ecuador: concentrations and mechanisms of mobility.Environ Earth Sci 2010;61(2):299–310.

De la Cruz-Reyna S. Random patterns of occurrence of explosive eruptions at Colimavolcano, Mexico. J Volcanol Geotherm Res 1993;55:51–68.

De La Cruz-Reyna S. Riesgos Geológicos: volcanes. Diagnóstico de Peligros e Identifica-ción de Riesgos de Desastres en México. Atlas Nacional de Riesgos de la RepúblicaMexicana. México D.F: CENAPRED-SINAPROC-SEGOB; 2001. p. 58–79.

Dobson PF, MahoodGA. Volcanic stratigraphy of the Los Azufres geothermal area, Mexico.J Volcanol Geotherm Res 1985;25:273–87.

Duffield W, Heiken C, Wohletz K, Maassen L, Dengo C, McKee E. Geology and geother-mal potential of the Tecuamburro volcano area of Guatemala. Geothermics1992;21:425–46.

Elders WA, Bird DK, Williams AE, Schiffman P. Hydrothermal flow regime and magmat-ic heat source of the Cerro Prieto geothermal system, Baja California, Mexico.Geothermics 1984;13(1–2):27–47.

Electroconsult. Campo Geot6rmico de Ahuachapin: Asistencia a La operaci6n del campo yestudio geológico de detalle. Milan, Italy, CEL final report AHU-D-5569/Rev.1; 1984.

Ellis AJ, Mahon WAJ. Chemistry and geothermal systems. New York: Academic Press;1977.

74 D.L. López et al. / Science of the Total Environment 429 (2012) 57–75

Eppler D, Baldridge S, Perry F, FloresW, Paredes JR. Geolog of the Azacualpa geothermalsite, departamento de Comayagua, Honduras, Central America. Los Alamos Nation-al Laboratory Report LA-10687-MS; 1987.

Esquivel AO. Diagnostico Nacional de Calidad Sanitaria de las Aguas Superficiales de ElSalvador, Programa de Descontaminacion de Areas Criticas, Ministerio del MedioAmbiente y Recursos Naturales de El Salvador. Contrato de Prestamo BIS-MARNNo 1209/OC-ES, San Salvador, El salvador; 2007.

Espindola JM, Macías JL, Tilling RI, Sheridan MF. Eruptive history of El Chichón volcano,Chiapas, Mexico. Effects on the Maya lowlands and Greenland ice cores. Bull Volca-nol 2000;62:90-104.

Fernandez-Turiel JL, Garcia-Valles M, Gimeno-Torrente D, Saavedra-Alonso J, Martinez-Manent S. The hot spring and geyser sinters of El Tatio, Northern Chile. SedimentGeol 2005;180:125–47.

Finch RC. Catalog of known hot springs and thermal place names for Honduras. Los Ala-mos National Laborator Report LA-10784-MS; 1987.

Foley D, Moore JN, Lutz SJ, Palma JC, Ross HP, Tobias E, Tripp AC. Geology and geophys-ics of the Zunil geothermal system, Guatemala. GRC Trans 1990;14:1405–12.

Gemmel BJ. Geochemistry of metallic trace elements in fumarolic condensates fromNicaraguan and Costa Rican volcanoes. J Volcanol Geotherm Res 1987;33:161–81.

Giggenbach WF. The isotope composition of waters from the El Tatio geothermal field,northern Chile. Geochim Cosmochim Acta 1978;42:979–88.

Giggenbach WF. Geothermal solute equilibria. Derivation of Na–K–Mg–Ca geoindica-tors. Geochim Cosmochim Acta 1988;52:2149–765.

Goff F, Shevenell L, Truesdell AH, Janik CJ, Grigsb CO, Paredes R. Hydrogeochemistryand prelimninary reservoir model of the Platanares geothermal system, Honduras,Central America. Los Alamos National Laboratory Report LA-UR-86-1404; 1986a.

Goff F, Grisby CO, Shevenell LA, Gutierrez JW. Geochemistry at Honduras GeothermalSites. Los Alamos Science; 1986b. p. 90–3. Fall.

Goff F, Truesdell AH, Janik C, Adams A, Roldan A, Meeker K. Hydrogeochemical explo-ration of the Tecuamburro volcano region, Guatemala. GRC Trans 1989;13:141.

Goff F, Janik C, Fahlquist L, Adams A, Roldan A, Revolorio M, et al. A re-evaluation of theMoyuta geothermal system, southern Guatemala. Los Alamos National LaboratoryReport LA-UR-91-1831; 1991.

Goldsmith LH. The Momotombo Geothermal Field, Nicaragua. Development History.Internal report. Berkeley, CA, USA: GEONOMICS, Inc.; 1975.

González Partida E, Torres Rodriguez V, Birkle P. Plio-pleistocene volcanic history of theAhuachapán geothermal system, El Salvador: the Concepción de Ataco Caldera.Geothermics 1997;26:555–75.

González EP, Tello EH, VermaMP. Geochemical and isotopic analysis of geothermal wa-ters and springs to define the state of water–rock equilibrium at the Los Azufresreservoir, Michoacán, México. Ing Hidraul Mex 2000;XV(3):89–99.

González EP, Tello EH, VermaMP. Interaction between geothermal water and springs at theLos Humeros geothemal field, Puebla, Mexico. Ing Hidraul Mex 2001;XV(2):185–94.

Hammarlund L, Piñones J. Arsenic in geothermal waters of Costa Rica. M. Sc. Thesis KTHLand and Water Resources Engineering; 2009.

Hammarlund L, Piñones J, Bhattacharya P, Mukherjee A, Nordstrom D, Bundschuh J,et al. Study of geothermal water–groundwater interaction and evolution in ther-mal fields of Costa Rica. Geol Soc Am Abstr Programs 2009;41(7):219.

Hart WJE. The panchimalco tephra, El Salvador, Central America. Rutgers University, M.Sc. Thesis; 1981.

Herrera J, Rocha V. Carta de manifestaciones termales en la República Mexicana.Informe 20/88, CFE, Gerencia de Proyectos Geotermoeléctricos; 1988.

Inguaggiato S,Martin-Del Pozzo AL, Aguayo A, Capasso G, Favara R. Isotopic, chemical anddissolved gas constraints on spring water from Popocatepetl volcano (Mexico): evi-dence of gas–water interaction between magmatic component and shallow fluids.J Volcanol Geotherm Res 2005;141:91-108.

Janik CJ, Truesdell AH, Goff F, Shevenell L, Stallard ML, Trujillo PE, et al. A geochemicalmodel of the Platanares geothermal system, Honduras. J Volcanol Goetherm Res1991;45:125–46.

Jasmin R, Williams-Jones AE, Clark Jr. Mineralization associated with scale and alteredrock and pipe fragments from the Berlín geothermal field, El Salvador; implicationsfor metal transport in natural systems. J Volcanol Geotherm Res 2005;145:81–96.

Kelly WR, Holm TR, Wilson SD, Roadcap GS. Arsenic in glacial aquifers: sources andgeochemical controls. Ground Water 2005;43:500–10.

KimM-J, Nriagu J, Haack S. Carbonate ions and arsenic dissolution by groundwater. En-viron Sci Technol 2000;34:3094–100.

Landrum JT, Bennett PC, Engel AS, Alsina MA, Pastén PA, Miliken K. Partitioning geo-chemistry of arsenic and antimony, El Tatio Geyser Field, Chile. Appl Geochem2009;24:664–76.

Lara F, Cornejo L, Yáñez J, Freer J, Mansilla H. Solar-light assisted arsenic removal fromnatural waters: the effect of iron and citrate concentrations. J Chem Technol Bio-technol 2006;81:1282–7.

Larocque ACL, Stimac JA, Siebe C, Greengrass K, Chapman R, Mejia SR. Deposition of ahigh-sulfidation Au assemblage from a magmatic volatile phase, Volcán Popocaté-petl, Mexico. J Volcanol Geotherm Res 2008;170:51–60.

Laughlin AW. Recommendations report for the Platanares geothermal site, Departmentof Copán, Honduras. Los Alamos National Laborator Report LA-11410-MS; 1988.

Lima Lobato EM, Palma J, RoldanManzo RR. Geothermal Guatemala, past, present and futureof geothermal energy in Guatemala. GRC Bulletin; 2003. p. 117–21. May/June.

Lippman MJ, Truesdell AH, Frye G. The Cerro Prieto and Salton Sea geothermal fields —are they really alike? Proc. 24th Workshop on Geothermal Reservoir Engineering.California: Stanford University; 1999. SGP-TR-162.

López DL, Ransom L, Perez N, Hernandez P, Monterrosa J. Dynamics of diffuse degassingat Ilopango Caldera, El Salvador. In: Rose WI, Bommer JJ, López DL, Carr MJ, MajorJJ, editors. Natural Hazards in El Salvador. Geological Society of America SpecialPaper No. 375; 2004. p. 191–202.

López DL, Ransom L, Monterrosa J, Soriano T, Barahona F, Olmos R, et al. Volcanic arse-nic and boron pollution of Ilopango Lake, El Salvador. In: Bundschuh J, ArmientaMA, Birkle P, Bhattacharya P, Matschullat J, Mukherjee AB, editors. Natural arsenicin groundwaters of Latin America — Occurrence, health impact and remediation.Taylor & Francis Group; 2009. p. 129–43.

López HA. Synthesis of the regional geology at the Las Tres Vírgenes geothermal zone,B.C.S., Mexico. Geotermia 1998;14:3-14.

Mambo VS, Yoshida M. Behavior of arsenic in volcanic gases. Geochem J 1993;27:351–9.Mansilla HD, Cornejo L. Relevamiento de Comunidades Rurales de América Latina para

la Aplicación de Tecnologías Económicas para potabilización de aguas: Chile. LaPlata, Argentina: Digital Grafic; 2002. p. 43–58.

McCutcheon M. Anthropogenic and volcanic pollution at Coatepeque Lake, El Salvador.Ohio University, Senior Thesis, B.Sc. in Geology. 1998.

Mazor E, Mañon AM. Geochemical tracing in producing geothermal fluids: a case studyat Cerro Prieto. Geothermics 1979;8:231–40.

Martínez-Estrella JI, Torres RJ, Iglesias ER. A GIS-based information system for moder-ate to low-temperature Mexican geothermal resources. Proc. of the World Geo-thermal Congress 2005, Antalya, Turkey, 24–29 April; 2005.

Mercado S, Bermejo F, Hurtado R, Terrazas B, Hernández L. Scale incidence on produc-tion pipes of Cerro Prieto geothermal wells. Geothermics 1989;18:225–32.

Montalvo F, Axelsson G. Assessment of chemical and physical reservoir parametersduring six years of production and re-injection at the Berlín geothermal field (ElSalvador). Kyushu-Tohuko, Japan: Proceedings of the World Geothermal Congress;2000. p. 1–6.

Mora JC, Jaimes-Viera MC, Garduño-Monroy VH, Layer PW, Pompa-Mera V, GodinezML. Geology and geochemistry characteristics of the Chiapanecan Volcanic Arc(Central Area), Chiapas Mexico. J Volcanol Geotherm Res 2007;162:43–72.

Ormachea M, Blanco E, Ramos O, García ME, Bhattacharya P, Thunvik R, et al. Arsenicoccurrence in thermal springs of the Central Bolivian Altiplano. In: Jean JS,Bundschuh J, Bhattacharya P, editors. Arsenic in Geosphere and Human Diseases,As 2010. In: Bundschuh J, Bhattacharya P, editors. Interdisciplinary Book Series:Arsenic in the Environment — ProceedingsCRC Press/Taylor and Francis; 2010.p. 520–2.

Parello F, Aiuppa A, Calderon H, Calvi F, Cellura D, Martinez V, et al. Geochemical char-acterization of surface waters and groundwater resources in the Managua área(Nicaragua, Central America). Appl Geochem 2008;23:914–31.

Pascua C, Charnock J, Polya DA, Sato T, Yokoyama S, Minato M. Arsenic-bearing smec-tite from the geothermal environment. Mineral Mag 2005;69:897–906.

Pérez NM, Salazar JML, Hernández PA, Olmos R, Barahona F, Cartagena R, et al. DiffuseCO2 and 222Rn degassing from San Salvador volcano, El Salvador, Central America.In: Rose WI, Bommer JJ, López DL, Carr MJ, Major JJ, editors. Natural Hazards in ElSalvador. Geological Society of America Special Paper No. 375; 2004. p. 227–36.

Planer-Friedrich B, Lehr C, Matschullat J, Merkel BD, Nordstrom DK, Sandstrom MW.Speciation of volatile arsenic at geothermal features in Yellowstone NationalPark. Geochim Cosmochim Acta 2006;70:2480–91.

Plan Piloto Oruro-003 (PPO-3). Hidrología del área del PPO. Ministerio de DesarrolloSostenible y Medio Ambiente. Swedish Geological AB, La Paz, Bolivia; 1996.

Porras EA, Tanaka T, Fujii H, Itoi R. Numerical modeling of the Momotombo geotermalsystem, Nicaragua. Geothermics 2007;36:304–29.

Portugal E, Birkle P, Barragán RM, Arellano VM, Tello E, TelloM. Hydrochemical–isotopicaland hydrogeological conceptual model of the Las Tres Vírgenes geothermal field, BajaCalifornia, Mexico. J Volcanol Geotherm Res 2000a;101:223–44.

Portugal E, Izquierdo G, VermaMV, Barragán RM, Pérez A, de León J. Isotopic and chem-ical behaviour of fluids of Cerro Prieto geothermal field, Mexico. Proc. 21st AnnualPNOC-EDC Geothermal Conference, Manila, Philippines; 2000b. p. 227–32.

Pokrovski GS, Kara S, Roux J. Stability and solubility of arsenopyrite, AsFeS, in crustalfluids. Geochim Cosmochim Acta 2002a;66:2361–78.

Pokrovski GS, Zakirov IV, Roux J, Testemale D, Hazemann JL, Bychkov AY, et al. Exper-imental study of arsenic speciation in vapour phase to 500 °C: implications for Astransport and fractionation in low-density crustal fluids and volcanic gases. Geo-chim Cosmochim Acta 2002b;66:3453–80.

Pullinger C. Evolution of the Santa Ana volcanic complex, El Salvador. Michigan Tech-nological University, M. Sc. in Geology, 1998.

Quijano-León JL, Gutiérrez-Negrín LCA. 30 years of geothermal-electric generation inMexico. GRC Bulletin; 2003. p. 198–203. Sept–Oct.

Quintanilla J, Ramos O, Ormachea M, García ME, Medina H, Thunvik R, et al. Arseniccontamination, speciation and environmental consequences in the Bolivian Pla-teau. In: Bundschuh J, Armienta MA, Birkle P, Bhattacharya P, Matschullat J,Mukherjee AB, editors. Natural arsenic in groundwaters of Latin America — Occur-rence, health impact and remediation. Taylor & Francis Group; 2009. p. 91–9.

Quintanilla J. Evaluación hidroquímica de la cuenca de los lagos Uru Uru y Poopó. LaPaz, Bolivia: IIQ-UMSA; 1994.

Ransom L. Volcanic diffuse soil degassing and lake chemistry of the Ilopango Calderasystem, El Salvador, Central America. Ohio University M.Sc. in Geology; 2002.

Ravenscroft P, McArthur JM, Hoque BA. Geochemical and palaeohydrological controlson pollution of groundwater by arsenic. In: Chappell WR, Abernathy CO, CalderonRL, editors. Fourth International Conference on Arsenic Exposure and Health Ef-fects. U.K.: Science Ltd. Oxford; 2001. p. 1-19.

Rittle KA, Drever JI, Colberg PJS. Precipitation of arsenic during bacterial sulfate reduc-tion. Geomicrobiol J 1995;13:1-11.

Romero L, Alonso H, Campano P, Fanfani L, Cidub R, Dadea C, et al. Arsenic enrichmentin waters and sediments of the Rio Loa (Second Region, Chile). Appl Geochem2003;18:1399–416.

Rouwet D, Taran YA, Inguaggiato S, Varley N, Santiago JA. Hydrochemical dynamics ofthe “lake-spring” system in the crater of El Chichon volcano (Chiapas, Mexico).J Volcanol Geotherm Res 2008;178:237–48.

75D.L. López et al. / Science of the Total Environment 429 (2012) 57–75

SEAN. Popocatépetl (México): increased fumarolic activity in summit crater. SmithsonInst (SEAN) Bull 1986;11:10–1.

Segovia N, Armienta MA, Seidel JL, Monnin M, Peña P, López B, et al. Radon in soil andchemical composition of spring water near the Popocatepetl volcano. Geofis Int2002;41:399–405.

Segovia N, Armienta MA, Valdes C, Mena M, Seidel JL, Monnin M, et al. Volcanic mon-itoring for radon and chemical species in the soil and in spring water samples.Radiat Meas 2003;36:379–83.

Sahai N, Lee YJ, Xu H, Ciardelli M, Gaillard J-F. Role of Fe(II) and phosphate in arsenicuptake by coprecipitation. Geochim Cosmochim Acta 2007;71:3193–210.

Signorelli S. Arsenic in volcanic gases. Environ Geol 1997;32:239–44.Shiraki R, Brantley SL. Kinetics of near equilibrium calcite precipitation at 100 °C: an

evaluation of elementary reaction-based and affinity-based rate laws. GeochimCosmochim Acta 1995;59:1457–71.

Smedley P, Kinniburgh DG. A review of the source, behaviour and distribution of arse-nic in natural waters. Appl Geochem 2002;17:517–68.

Sracek O, Bhattacharya P, Jacks G, Gustafsson JP, Brömssen von M. Behavior of arsenicand geochemical modeling of arsenic enrichment in aqueous environments. ApplGeochem 2004;19(2):169–80.

Sullivan M.Water balance of the San Simon river groundwater basin, El Salvador, CentralAmerica: implications for the Berlín geothermal field. Ohio University, M. Sc. in Geol-ogy; 2008.

Swedlund PJ, Webster JG. Adsorption and polymerisation of silicic acid on ferrihydrite,and its effect on arsenic adsorption. Water Res 1999;33:3413–22.

Taquichiri L, Llanque E, Cayetano M, Zabala M. Junt'uma, Aguas Termales en el Depar-tamento de Oruro: Un Potencial Ignorado, Centro de Ecología y Pueblos Andinos(CEPA). Oruro, Bolivia: Latinas Editores; 2005.

Taran Y, Fischer TP, Pokrovsky B, Sano Y, Armienta MA, Macías JL. Geochemistry of thevolcano-hydrothermal system of El Chichón Volcano, Chiapas, Mexico. Bull Volca-nol 1998;59:436–49.

Taran Y, Gavilanes JC, Cortés A, Armienta MA. Chemical Precursors to the 1998–1999eruption of Colima volcano, Mexico. Rev Mex Cienc Geol 2000;17:112–26.

Taran YA, Bernard A, Gavilanes JC, Lunezheva E, Cortés A, ArmientaMA. Chemistry andmin-eralogy of high-temperature gas discharges from Colima volcano, Mexico. Implicationsfor magmatic gas–atmosphere interaction. J Volcanol Geotherm Res 2001;108:245–64.

Taran Y, Rouwet D, Inguaggiato S, Aiuppa A. Major and trace element geochemistry ofneutral and acidic thermal springs at El Chichón volcano, Mexico. Implications formonitoring of the volcanic activity. J Volcanol Geotherm Res 2008;178:224–36.

Tassi F, Vaselli O, Capaccioni B, Macías JL, Nencetti A, Montegrossi G, et al. Chemicalcomposition of fumarolic gases and spring discharges from El Chichón volcano,

Mexico: causes and implications of the changes detected over the period 1998–2000. J Volcanol Geotherm Res 2003;123:105–21.

Tassi F, Aguilera F, Darrah T, Vaselli O, Capaccioni B, Poreda RJ, et al. Fluid geochemistryof hydrothermal systems in the Arica-Parinacota, Tarapacá and Antofagasta re-gions (northern Chile). J Volcanol Geotherm Res 2010;192:1-15.

Todesco M, Rutqvist J, Chiodini G, Pruess K, Oldenburg CM. Modeling of recent volcanicepisodes at Phlegrean Fields (Italy): geochemical variations and ground deforma-tion. Geothermics 2004;33:531–47.

Torres RJ,Martínez-Estrella JI, Iglesias ER. Database ofMexicanmedium- to low-temperaturegeothermal resources. Proc. of the World Geothermal Congress 2005, Antalya, Turkey,24–29 April; 2005.

Torres VR, Venegas SS, Herrera JF, González EP. Manifestaciones termales de la RepúblicaMexicana. In: Torres VR, editor. Geotermia en México. Universidad Autónoma deMéxico; 1993.

Varekamp JC, Luhr JF, Prestegaard K. The 1982 euprtions of El Chichón volcano (Chiapas,Mexico): carácter of the eruptios, ash-fall deposits, andgas phase. J Volcanol GeothermRes 1984;23:39–68.

Viggiano-Guerra JC, Sandoval-Medina F, Flores-ArmentaMC, Pérez RJ, González-Partida E.Application of SPCALC for the chemical and thermodynamic fluidal speciation: Casestudy for the wells LV-4A, LV-11 and LV-13 from the Las Tres Vírgenes geothermalfield, BCS. Geotermia 2009;22:12–27.

Webster JG, Nordstrom DK. Geothermal Arsenic. The source, transport and fate of arse-nic in geothermal systems. In: Welch AH, Stollenwerk KG, editors. Arsenic inGroundwater: Geochemistry and Occurrence. Kluwer Academic Publishers; 2003.p. 101–26.

Werner C, Janik CJ, Goff F, Counce D, Johnson L, Siebe C, et al. Geochemistry of summitfumarole vapors and flanking thermal/mineral waters at Popocatépetl volcano,Mexico. Los Alamos: Los Alamos National Laboratory Report LA-13289-MS;1997.

Wilkie JA, Hering JG. Rapid oxidation of geothermal arsenic(III) in stream waters of theEastern Sierra Nevada. Environ Sci Technol 1998;32:657–62.

Williams H, Meyer-Abich H. Volcanism in the southem part of El Salvador with partic-ular reference to the collapse basins of lakes Coatepeque and Ilopango, 32. Univer-sity of California Publications in Geological Sciences; 1955. p. 144.

Williams H, McBirncy A, Dengo G. Geologic reconnaissance of southeastern Guatemala.Univ of Calif Pubs in Geol Sci; 1964. p. 50.

Yañez J, Mansilla H, Figueroa L, Cornejo L. Arsenic speciation in human hair: a new per-spective for epidemiological assessment in chronic arsenisism. J Environ Monit2005;7:1335–41.